Bay's head and right shoulder appeared to strike the wall after he dove onto the warning track in an attempt to catch Jay Bruce's inside-the-park home run leading off the second. Bay did not move for several moments, before eventually getting to his feet and throwing in the ball.
Justin Turner, who went 1-for-4 in his second rehab game with Triple-A Buffalo on Friday night, will come to Citi Field on Saturday in anticipation of a roster move to place Bay on the disabled list.
Bay missed the final two months of the 2010 season with a severe concussion after slamming into Dodger Stadium's left-field wall. He also missed six weeks earlier this year with a fractured left rib, the result of a diving play at Citi Field.
"I felt terrible for Jason," manager Terry Collins said. "As he always does, he plays the game only one way -- all out. Never backed off anything, didn't back off the wall, didn't back off the broken rib. He still dove and still played the game the only way he knows how."
Bay told Collins he felt dizzy in the moments after the collision. He was taken out of the game and was replaced by Scott Hairston, who belted a two-run homer in the seventh.
Collins said Bay's situation is now similar to that of Mets catcher Josh Thole, who landed on the seven-day disabled list earlier this season with a concussion. Thole had previously sustained a concussion two years prior playing for Triple-A Buffalo.
"It wasn't that this one was more serious than the others, but since [Bay's] had it in the past, this becomes a pretty serious thing," Collins said.
Bay is batting .187 with four home runs in 22 games.