The Red Sox will place pitcher Brian Johnson on the disabled list with "elbow tightness," according to Alex Speier:
Brian Johnson will be placed on DL due to elbow tightness. Evaluation coming today/tomorrow. Left Sunday's game after 4 IP, 60 pitches— Alex Speier (@alexspeier) August 3, 2015
It's an ominous symptom, that. There are many instances of elbow tightness that are not followed by Tommy John Surgery, but few Tommy John Surgeries that don't start with elbow tightness.
The Red Sox do have depth in terms of even their upper-level pitching prospects, as made obvious by Henry Owens being scheduled to start tomorrow's game. But that doesn't really make any significant injury to Johnson any better. For one thing, his absence very likely means that Joe Kelly will survive in the rotation until the end of the season, which is pretty much the worst.
For another, if Johnson is down for a while, it gives the Red Sox that much less flexibility to work with headed into the offseason. It would mean he could no longer be used as a trade piece, and would make trading Henry Owens that much more difficult given the lack of a backup option.
No, the Red Sox are in no shape to be losing the likes of Johnson right now. Really, they're in no shape to be losing much of anything. Hopefully this doesn't turn out to be as bad as it could be, and as bad as the recent history of pitching prospects in this league makes you think it might be.