Bay was removed from Friday night's lineup after he continued to experience soreness in his right shoulder, which he jammed trying to field a ball hit by Phillies second baseman Placido Polanco on Tuesday in Philadelphia.
Bay says he'll be ready to play Monday when the Mets play a doubleheader versus the Florida Marlins after getting a couple extra days of extra rest after Saturday and Sunday's games against the Atlanta Braves were canceled due to Hurricane Irene.
Bay ended up playing Wednesday night after taking a couple anti-inflammatories, but the soreness remained. He hoped it would go away during the off day on Thursday. It didn't.
"It's still sore today," Bay said. "The doctor wanted me to undergo an MRI to make sure (there was no structural damage). ... Yeah, I'll be ready for Monday."
Bay is hitting .233 this season with nine home runs and 43 RBIs.
Jason Pridie replaced Bay in Friday's lineup, and is batting eighth.
The Mets made the moves before Friday night's game against Atlanta.
Hairston's move was retroactive to Aug. 24, a day after he strained his left oblique while taking a swing.
The 40-year-old Batista began the season with St. Louis and was 3-2 with a 4.60 ERA in 26 games, including one start. He joined the Mets in July and went 3-0 with a 4.24 ERA at Buffalo.
Batista is set to pitch for his 10th team in the majors. The Mets need a spot starter next week and may use Batista.
Mike Mazzeo is a frequent contributor to ESPNNewYork.com. Information from The Associated Press was used in this report.