It didn't look too bad at first; the linked article above says:
X-rays taken on Bay's ribcage were negative, though the outfielder may receive a precautionary MRI exam on Tuesday, and it sounded doubtful that he would be in the lineup.
"I can't tell you how I'm going to feel tomorrow," Bay said after Monday's game. "Given the nature of it, I'll probably be pretty sore. But I may be all right."
Apparently, he wasn't all right:
Mets announce Jason Bay to DL (nondisplaced fracture of rib). Zach Lutz recalled— DKnobler (@DKnobler) April 24, 2012
Lutz is a third baseman who has been bumming around the Mets' farm system for six seasons, never doing much of anything special. He's hitting .333/.425/.556 this season in 74 plate appearances, so Sandy Alderson probably figured, "Why not?"