5 Undervalued Hitters and Fantasy Baseball Waiver Wire Adds (Week 18) - Players Who Are Underowned

August 3, 2025

Five fantasy baseball waiver wire pickups, sleepers, and trade targets for Week 18 -- undervalued hitters outperforming their rostership and should be more widely owned.

As July winds down and Thursday's MLB trade deadline looms, it's as crucial a time as ever for fantasy managers. So, before your fantasy football drafts begin, don't forget to make the right moves on the baseball side of things.

The waiver wire becomes even more critical in August, and Royce Lewis, Ivan Herrera, Ryan McMahon, Otto Lopez, and Chandler Simpson are all viable options to consider for the stretch run.

They each bring something unique to the table, and it may be the difference between your team and a playoff spot.

Royce Lewis, 3B, Minnesota Twins

59% rostered

Is Royce Lewis the new Byron Buxton? Both are former first-round draft picks of the Twins, incredibly talented, but have trouble staying healthy. Lewis has yet to play over 100 games in a season since he debuted during the 2022 campaign, while Buxton is on pace to appear in over 100 contests for just the third time in his 11-year MLB career. It's a shame because Minnesota is much better when both of these players are on the field.

Over his last seven games, Lewis is slashing .423/.464/.692 with a home run, five RBI, two walks, and a stolen base. His current hot streak has brought his season totals to a .250 batting average and .704 OPS alongside 15 extra-base hits, 20 RBI, 15 walks, and 24 strikeouts. The 26-year-old has already dealt with two significant IL stints this season, both due to a left hamstring strain. Because of this, Lewis has only played in 52 games as of Tuesday.

A 451-foot solo shot from Royce Lewis 😮 pic.twitter.com/7huFGhBcMx

— MLB (@MLB) July 20, 2025

Since being activated from the injured list on July 1, Lewis is 22-for-73 with six doubles, three homers, and 11 RBI. FanGraphs projects a 109 wRC+ and .327 wOBA for him the rest of the season. Additionally, he has a .291 xBA and .488 xSLG, per Statcast. While the risk in rostering him is high, so is the reward.

Ivan Herrera, C, St. Louis Cardinals

55% rostered

One of the most underrated catchers in the league, Herrera is batting .306 with a .874 OPS across 55 games this season, alongside nine home runs, 39 RBI, 24 walks, and 43 strikeouts. Since moving Willson Contreras to first base full-time, the 25-year-old has filled in nicely behind home plate. He has a career .296/.375/.433 slash line with 37 extra-base hits over 151 games since making his MLB debut in May 2022.

Iván Herrera's 3rd hit of the night goes a LONG WAY! pic.twitter.com/d7ux1y9Ms6

— St. Louis Cardinals (@Cardinals) July 25, 2025

Outside of a few stints on the injured list this year, he's been arguably the most consistent member of the Cardinals when he does step onto the batter's box. Herrera had a .923 OPS in the first half and may have been an All-Star if it weren't for knee inflammation sidelining him for almost a month. He has just a .687 OPS in the second half as well, but has appeared in only 11 games thus far since the break due to a left hamstring strain that kept him out until July 13.

Herrera has performed exceptionally well against lefties, particularly, with a .346 average and 1.220 OPS in 52 at-bats when facing southpaws. However, he's been just as successful versus righties as well, batting .294 with eight extra-base hits. He's also been one of St. Louis' most productive bats with runners in scoring position, slashing .400/.458/.640 with 31 RBI. The backstop has begun playing in the outfield and DHing for the Cards, too, so he should offer more positional flexibility soon.

Ryan McMahon, 3B, New York Yankees

44% rostered

The Colorado Rockies finally pulled the trigger on dealing their longtime third baseman, trading McMahon to the Yankees on July 25. New York was in desperate need of help at the hot corner, and so far, the one-time All-Star has delivered. The 30-year-old has gone 4-for-12 in four games since the trade, with a double, three RBI, two walks, and five strikeouts.

Ryan's 1st Ribbies in Yankees Pinstripes 👊 pic.twitter.com/hMQvWayR1C

— New York Yankees (@Yankees) July 27, 2025

That said, McMahon did benefit from playing his home games at Coors Field for almost nine seasons. In 2025, he had an .856 OPS with seven doubles, 11 home runs, 22 RBI, and 58 strikeouts at home, while he had just a .589 OPS, five homers, 13 RBI, and 75 strikeouts on the road. Luckily, Yankee Stadium is still a hitter-friendly park, and the team is likely hoping that McMahon can find similar success in the Bronx.

He's still seeking his first long ball in pinstripes, but McMahon had 16 before the trade, and it likely won't be much longer until he takes advantage of the short porch. However, his power was about all he'd done well offensively to this point in the season, as he's batting .221 with 132 strikeouts and a 93 wRC+ in 104 games on the year. Perhaps playing on a contender for the first time in his career will give McMahon a boost at the plate. Although he doesn't necessarily move the needle for the Yanks, he does offer a solution to their third base conundrum for the time being.

Otto Lopez, 2B/SS, Miami Marlins

42% rostered

Lopez continues to have a solid season for Miami, setting new career highs in nearly every offensive category, including home runs (11), RBI (55), and walks (34) across 90 games this year. Although his batting average (.249) is down from 2024 (.270), his on-base percentage and slugging percentage have risen, culminating in a higher OPS. He's notched 25 extra-base hits, just five shy of his total from last season in nearly 30 fewer contests.

Likewise, he's 10 stolen bases away from matching his 20 from last season. The 26-year-old has seemingly found his power stroke, now whether he can pair that with his contact skills and speed remains to be seen. If the Dominican infielder can find a way to put it all together offensively, he'd be a strong building block for the Marlins moving forward.

Otto Lopez makes it 3-0 @Marlins! pic.twitter.com/Dv5sbPeqAO

— Marlins Radio Network (@MarlinsRadio) July 30, 2025

As we addressed in Week 16 of this column, it's been a long journey for Lopez to reach this point. He made his MLB debut in August 2021, but didn't exceed rookie limits until 2024. He was initially signed by the Toronto Blue Jays in 2016 and was designated for assignment by the club in February 2024. Lopez was then briefly with the San Francisco Giants before being picked up by the Marlins on waivers, where he's been since April 2024. Being able to stick with one team has helped Lopez find his groove.

Chandler Simpson, OF, Tampa Bay Rays

42% rostered

Simpson has appeared a few times in this column throughout the season, primarily for his speed and propensity to steal bases. Through 63 major league games, Simpson has 32 stolen bases, which ranks third in baseball and sets a new franchise record for Rays rookies, with his sprint speed placing in the 99th percentile in MLB, per Statcast. Moreover, the 24-year-old has struck out just 21 times in 241 plate appearances, with his 11.6 whiff percentage and 8.8 strikeout rate placing in the 98th percentile.

Chandler Simpson has the most stolen bases in Rays rookie history.

He has played 57 games. pic.twitter.com/aWjHxGanpe

— Jake (@TBRaysCentral) July 23, 2025

He's batting .300 this season with 67 hits, only nine of which have gone for extra bases -- eight doubles and a triple. Yes, Simpson has yet to launch his first big league homer, but considering he had just one in nearly four minor league seasons, it's clear that Simpson is lacking in power. He has a paltry .345 slugging percentage and struggles to draw walks, with only 13 in 241 plate appearances.

Simpson doesn't make a lot of hard contact or drive the ball out of the ballpark, but his elite speed and bat-to-ball skills allow him to be more than serviceable at the MLB level and with fantasy teams. Just don't expect him to turn into Aaron Judge or Kyle Schwarber anytime soon.

More Fantasy Baseball Advice