This is an inflammation of the tendon that connects the biceps to the shoulder. Via a Mets Today blog post from 2010, we find that this is a warning sign of a significant arm issue. Jeff Luhnow certainly doesn't seem to be too bothered, saying he's "not overly concerned...we believe this is something that with a few days of rest and rehab he'll be back in action soon."
Feldman threw 181.2IP in 2013 - his highest number since throwing 189.2IP in 2010 (of course there were relief stints in between). And of course he looked perfectly fine in his first three starts of the season, in which he allowed 7H/1ER in 20.2IP.
So I guess we hope for the best with Feldman as the anchor of the rotation.
It is telling that the Astros didn't just shift Jerome Williams over to his spot. McHugh has thrown 14IP, allowing 12H/7ER, 8K:5BB. In two "official" starts (piggyback, and all) he has thrown 9IP, allowing 8H/3ER, 6K:2BB and is coming off an April 17 start at Colorado Springs in which he threw 5IP, 3H/1ER, 3K:0BB and 41 of his 56 pitches for strikes. The fun part is that, should McHugh win, it'll be his first career Major League win.