Czech reporter Roman Jedlicka said via Twitter that Rask, playing for Plzen, left the game after one period.
"I slightly pulled my groin, it is not serious," Rask told Czech TV, according to Jedlicka's Twitter account. "I had groin problems last year so I took [a] little rest just for precaution."
Rask was sidelined for the Bruins from March into April with a groin/abdomen injury. He was 11-8-0 with a 2.05 goals-against average and .929 save percentage in 23 games last season.
Rask, 25, signed a one-year contract with the Bruins on July 1.