Grandal was batting for the second time as part of the Padres' five-run inning when he stepped out of the batters' box after swinging and missing the first and only pitch Simon threw to him.
San Diego head trainer Todd Hutcheson and manager Bud Black attended to Grandal, who took a swing before opting to leave.
"I don't think it's that bad," said Grandal after the Padres' 11-5 victory over the Reds. "We're going to see what happens tomorrow. But I think I'll be OK."
Grandal was replaced by catcher John Baker, who completed his at-bat with a fly-ball out to left field.
"It just depends on the degree of the strain," said Black when asked about the severity of the injury. "We'll deal with it."
Grandal, who walked in his first plate appearance earlier in the inning, is hitting .312 with five home runs and 15 RBIs. The game Monday marked exactly month in the big leagues, as he earned a promotion on June 30 from Triple-A Tucson.
"He's played very well for us," said Padres third baseman Chase Headley. "But sometimes it can be a long time [disabled list], but sometimes it's only a couple of days. I hope it's only a couple of days."
The Padres are short on depth in the Minor Leagues as two catchers -- Nick Hundley and Ali Solis -- are on the disabled list. However, Hundley was close to coming off the DL with Tucson and could be a candidate to join the team if Grandal lands on the DL.