Reina, deputy to first-choice shot-stopper Manuel Neuer, picked up the injury in Bayern's game with a Paulaner dream team of amateur footballers in Unterhaching.
The Bundesliga champions posted on their official Twitter page: "@PReina25 [Reina] has suffered an injury in his left calf muscle. There will be a more detailed diagnosis tomorrow."
Captain Philipp Lahm and first-team regulars Xabi Alonso and Thomas Muller were among those involved in the game, which Bayern won 5-1.
However, the match could prove to be a costly one for the Bavarians depending on the severity of Reina's injury.
Experienced third-choice keeper Tom Starke will likely serve as Neuer's number two should Reina be forced to spend any time on the sidelines.
Bayern's next competitive game comes a week on Saturday when they welcome Werder Bremen to the Allianz Arena.