As we enter the All-Star break this weekend, it's time to take a hard look at your roster(s) and assess your position for the second half. If you're near contention, finding the right player to help you make that final surge can make all the difference. Ask those who added Trea Turner or Gary Sanchez last season. Likewise, cutting ties with players bound to decline can be equally important. Some of this week's picks may seem surprising, but rest assured there are valid reasons behind each one.

Scouting players who may be undervalued and re-assessing players who may be overvalued is a weekly exercise that you should pursue. Fortunately, I have you covered. Here are a few players who are good buy or sell candidates based on their current performance compared to ownership levels. This could mean scooping them off waivers if possible or actively seeking a trade to acquire or discard certain players in order to maximize value.

I will include one player at each key position group (Infield, Outfield, Pitcher). Ownership levels are taken from Yahoo! Fantasy Sports.

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Underrated Players - Week 15

Matt Holliday (1B/OF, NYY) 63% owned

Holliday proved to be a draft-day steal and basically came for free in the latter rounds or off the waiver wire. There has still been some skepticism based on age and now he finishes the first half on the disabled list. In reality, he isn't injured at all. Holliday has been fighting a viral infection and should be back on the active roster this weekend. If he merely keeps at his current pace, Holliday could finish with a 30-HR, 100-RBI season in the Bronx. Considering the rate at which balls are flying out of Yankee Stadium and the quality of the surrounding cast, there's plenty of promise for him to do just that. If he's been dumped in your league, act now to grab him.

Domingo Santana (OF, MIL) 63% owned

Like the veteran Holliday, 24-year-old Santana is also on pace for 30 homers and 100 RBI in just his second season as a starter. He's shown an affinity for taking walks (12.3% BB%) which has led to a .383 OBP. He'll continue to swing and miss quite a bit, evidenced by a 71.8% contact rate and 13.3% swinging strike rate, but he makes the most of his contact when he does connect. Two more hits on Friday brought his season average up to .291, so labeling him a slugger is an outright injustice. Don't forget the nine steals as well, giving him a good chance to go 20/20 by year's end. Frankly, it's shocking that he's not owned in all leagues of 12 or more teams by this point. If he's somehow available in your league, make room on your roster for him. If you're in a keeper league, try to swing a deal for him before his true value is realized.

Julio Teheran (SP, ATL) 83% owned

It would seem Teheran fits better on the list below, but there's a case to be made for buying low on him now. Simply put, Teheran hates the Braves' new digs. His ERA at the new SunTrust Park, which has proven to be fairly hitter-friendly, is now at 7.58 over 46 1/3 innings. He's allowed almost twice as many home runs at home (13) than on the road (seven) in fewer innings. While he's under team control a couple more years, there have recently been rumors that the Braves may look to move the struggling young starter to further the rebuilding process. Bartolo Colon has been ditched already, Jaime Garcia may follow soon and R.A. Dickey won't be around much longer. The Braves have plenty of young arms in their farm system to get the next generation underway, so it makes sense that they would trade assets they don't need (Matt Adams and Jim Johnson, ahem) in order to accumulate talent. If Teheran moves to a contender, especially somewhere like Houston or New York, his stock will shoot up in a big way, as will the wins. He isn't an ace and likely never will be, but last year showed us what type of talent he does possess as an SP2. Buying low now from a frustrated leaguemate could prove fruitful come August.

Overrated Players - Week 15

Mike Zunino (C, SEA) 42% owned

You can't be blamed for snatching Zunino off waivers after his mid-June homer spree. Since then, he's been consistently going deep - once a week to be exact. That won't get the job done and neither will a .230 average or .296 OBP. That batting average is by far a career high mark for him, as is his 38.4% K% which keeps rising. Zunino is looking more and more like a younger version of Derek Norris, which isn't a compliment. Rather than sticking with Zunino, I prefer someone like Austin Hedges who has nearly identical stats but is still rounding into form and should eventually surpass him in average.

Jacoby Ellsbury (OF, NYY) 39% owned

Ellsbury is off the DL and will be playing full-time again with the injury to Aaron Hicks. This may seem like the right time to make him a bench stash in deeper leagues, but what exactly are we hoping for with Ellsbury? For a player who hasn't reached double-digit homers the last two seasons and has stayed around 20 SB, at best you could hope for 5 HR and 10 SB throughout the second half. That also comes with a .263 average that is consistent with his three-year average in New York. All that is assuming he even keeps the job when Hicks returns. There simply isn't enough upside here to warrant a single dollar of FAAB. If you're in a league that starts four or five outfielders, don't pay for past performance. Grab someone like Raimel Tapia or Manuel Margot instead.

Adam Wainwright (SP, STL) 62% owned

Look at this stat line and decide if it's the kind of pitcher you want on your fantasy squad: 5.90 ERA, 1.42 WHIP, 6.7 K/9. That's actually the line of Josh Tomlin, who is 10% owned. Now, look at Waino's: 5.48 ERA, 1.55 WHIP, 8.17 K/9. A few more strikeouts and he's up to nine wins now, but those ratios are killing your team if you drafted him. Reputation and track record have kept Wainwright on many rosters, but the 35-year-old is following up last year's disappointing season with another one. His 4.36 SIERA is just a shade above last year's 4.31 and most of his peripherals are just the same. There have been several occasions lately when it looks like the Wainwright of old is still there, but every other week he comes out with a start that completely devastates his ratios again (and his fantasy owners). Since June began, he's allowed more than five runs on three different occasions, including two starts with nine ER. He can still be streamed, preferably at home where he holds a 3.42 ERA, but he's become just as risky as a rookie pitcher like Dinelson Lamet, who possesses higher upside at this point.

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