Johan Santana may not pitch Thursday after all -- or the remainder of this season.

After Monday's 3-1 loss to the Rockies, Terry Collins revealed that said Santana's lower back stiffened considerably Monday. He is due to have an MRI on Tuesday.

Collins said Santana has not officially been scratched yet. But given the Mets' debate about whether to allow him to proceed in the first place, perhaps it's now inevitable the southpaw is shut down for 2012.

"He came in stiffer today than he had been," Collins said. "Actually, they got him loosened up. He threw his pen. He felt fine. When he was done, he got a little tight. He had the doctor look at it. And the doctor thought they should look at it tomorrow. So they're going to do that."