Marcum was supposed to start tomorrow's game against the Cardinals -- Jeremy Hefner will start instead.
Sandy Alderson told reporters the cortisone shot was "precautionary," and Marcum is expected to resume throwing in a couple of days.
Update - 9:36 pm: Terry Collins told reporters Marcum suffers this injury every spring, and expects him to start either on March 25 or 26.
6:35 pm: Seriously, which primary player on this team is actually healthy right now? The injuries have been unbelievable lately. Hopefully, Marcum can get through this in short order, but shoulders are tricky, and impingements in that joint are often mysterious. It's worth noting that Marcum was shut down for a period of time after his first spring start so he could build arm strength, and I wonder if these two incidents are related...
It's still unclear if Marcum will be ready to start the season. He hasn't pitched that much as it is, and this injury has to set him back no matter what. The Mets are reportedly stretching out Aaron Laffey, and so he could be one alternative to Marcum. Collin McHugh might also be an option, although he was recently sent to minor league camp. He was having a rough spring before being sent down, and he clearly needs to work on his stuff before he can be an effective big league pitcher.
In any case, I don't see the Mets promoting Zack Wheeler to fill-in for Marcum. He's close but needs more time at Triple-A, and promoting him now (as opposed to after the 20th day of the season) would start his free-agent clock, and the Mets would lose a year of his control as a result.