T.R. Sullivan reports that it is a tendon strain that Adam Eaton suffered today.
That's the same injury that sidelined him for three weeks in August last year, and that resulted in him being limited the rest of the way (the Padres opted to go with Pedro Astacio instead of Eaton in their playoff rotation last year, which should tell you all you need to know about how limited he was).
He's scheduled to have an MRI, but it sounds like this could be anything from a couple of weeks to a couple of months that he'll be on the shelf...
Hopefully, Will Carroll will have more tomorrow in his UTK...