Pete Grathoff of the Kansas City Star reported Perez's injury, adding the Royals will re-evaluate him in 48 hours to determine a potential roster move.
Grathoff added Royals manager Ned Yost is "relieved" Perez didn't suffer an oblique strain, which typically has a "recovery time of about 8 weeks" and an intercostal strain could have "half" of that recovery time.
Perez was removed from Friday's game after six innings. He appeared to be wincing in pain following a strikeout against Mariners starter James Paxton in the bottom of the sixth.
Royals backup catcher Drew Butera took over for Perez behind the plate in the top of the seventh inning. Butera is also in the team's starting lineup for Saturday's game, hitting eighth.
Losing Perez for an extended period of time would be a huge loss for the Royals, who trail the Cleveland Indians by 3.5 games in the American League Central and are in the second AL wild-card spot.
The 27-year-old Perez was the AL starting catcher in the All-Star Game. He ranks second on the Royals with 21 home runs and a .510 slugging percentage.