"He's concerned about it being something he's going to need some medication for," said Mets manager Terry Collins.
Bay, who is batting .357 with three home runs and seven doubles over his past 16 games, is merely the latest in a string of Mets players to suffer from illness; pitchers Chris Capuano and Tim Byrdak have both complained of symptoms in recent days.
The Mets did receive a boost Tuesday from the return of outfielder Angel Pagan, who missed one game with a sore quadriceps muscle. He collected three hits and drew a go-ahead walk in the seventh inning of the Mets' 11-6 loss to the Cardinals.
Though Collins called Pagan's injury "a small issue," he noted that the training staff will "certainly have to keep an eye on him" throughout the final nine games of the season.
Pagan, who entered batting .269 in September, has not hit a home run in a month and has more than doubled his career total with 10 errors in center field.