From the Globe's Extra Bases blog-type-thing:
Here's the scoop on Keith Foulke as we know it. The reason he's been held back in spring training is because he's been undergoing two weeks of "injections" to his knees.
This was confirmed by both Sox GM Theo Epstein and Foulke's agent, Dan Horwitz. Neither would explain what was in the injections due to confidentiality issues, though Horwitz said they were not cortisone. Epstein said the injections were to promote healing of the knees. Foulke had arthroscopic surgery on his left knee in July and on his right knee after last season.
"He'll be held back while this process is going on, but once it's over he should be fine," Epstein said.
Foulke threw off flat ground for about 10 minutes today and showed decent velocity. He was watched by a member of the Sox training staff the whole time. At the end of the session, Foulke was showing the trainer his push-off leg, and then went off into the trainer’s room while the rest of the pitchers and positional players worked out.
I am telling you, I will throw a tantrum. I will lay down on the floor and kick and scream. Do not tempt me.