Derek Holland threw a complete-game shutout against the Indians his last time out and struck out seven in five and two-third innings in the start before that (though he allowed five runs). With his top-prospect pedigree, increased average fastball speed (93.44 mph vs. 92.1 last year) and continued seasoning, he's the type of player who could put it all together in the second half.
Grady Sizemore's back again from the DL, but it's hard to get too excited about it. He's hitting .266 in 109 at-bats, and given his track record (.272 career average), vast improvement should not be expected. Sizemore's attempted only one steal (unsuccessfully), and that part of his game is likely to be greatly diminished if not gone entirely. What's left is a player with good but not great power - think Juan Rivera if Rivera were to have a full season of at-bats - poor plate discipline (5:33 K:BB ratio) and high injury risk.
Carlos Marmol was excellent last year despite an ungodly 52 walks in 77.2 IP largely because his strikeout rate (16 K/9) was off the charts. This year, he's cut down on the walks slightly (14 in 27 IP), but his strikeouts have also dipped to a merely elite 12.3 K/9. He's also blown his last two save chances and seen his average fastball velocity dip from 94.1 last year to 91.6 this season. It's an open question whether he can get away with the poor control at his current capacity, and that the Cubs are dysfunctional and playing poorly probably doesn't help.
Jonny Venters didn't win the closer job out of spring training, but he's the rare middle reliever that's valuable in nearly any format. Venters' 35:11 K:BB ratio in 35.2 IP is solid but unspectacular for a late-inning reliever, but his unheard of 9.29 to 1 GB:FB ratio sets him apart. Of the 16 hits he's allowed on the year, only two (one double and one triple) have gone for extra bases. He's allowed only two earned runs so far this season, while posting four wins and one save. Typically relievers don't throw enough innings to move the needle in ERA or WHIP, but when the former is 0.50 and the latter is 0.757, their impact is significant. Put differently, subtract Venters' numbers from C.C. Sabathia's (93.1 IP, 88 hits, 26 walks, 29 ER, 70 K, 2.80 ERA, 1.221 WHIP, 7-3 record), and you get a pitcher who is 3-3 with 35 strikeouts in 57.2 IP with a 4.21 ERA, a 1.510 WHIP and minus-one save, i.e., someone below replacement value in many formats.