Mathis, who has broken his wrist and face before, said this injury was more painful.
"I guess I underestimated the lack of mobility I've had for so long," Mathis said. "When I took the sling off, it was amazing how much atrophy took place in my shoulder, and that's been the biggest grind is just getting the strength back."
The 30-year-old was reinstated from the disabled list prior to Tuesday's series opener against the Reds after rehab-assignment games for both Double-A Jacksonville and Triple-A New Orleans.
Acquired in the big offseason trade with Toronto, Mathis was expected to be Miami's primary backup catcher. The Marlins will carry three backstops for now: Mathis, Rob Brantly and Miguel Olivo.
"It's nice to be back on the field," said Mathis, who doesn't plan on wearing additional padding. "I've been playing down there [in the Minors] for about 10 days, and it's nice to be between the lines and back playing ball and participating again."