Bay suffered the concussion last Friday in Los Angeles when he made a running catch against the wall. Doctors diagnosed the concussion on Tuesday, starting a 48- to 72-hour clock that could land Bay on the disabled list.
Mets manager Jerry Manuel said that if there is no meaningful improvement by Thursday or Friday at the latest, Bay will be put on the 15-day DL.
"If there is nothing in terms of progress, we'll make a decision to put him on the DL," Manuel said before Wednesday's Mets-Cardinals game.
Manuel said Bay told him he did not feel much better Wednesday than he did Tuesday. Bay has never had a concussion diagnosed before.