Mathis broke his right collarbone on Feb. 23 in the Marlins' first Spring Training game.
"Basically, he didn't have any Spring Training," Marlins manager Mike Redmond said. "He got hurt the very first game of Spring Training. He never gave himself a chance to get going. It's going to be up to him for how long it takes for him to get his legs under him and get in shape."
Mathis opened the season on the disabled list, and by rule, he can spend 20 days on rehab assignment before the team decides either to activate him or keep him on the DL.
The short-term plan is two games behind the plate and the third game as DH. Mathis also has to build up to catching full games.
"Being a catcher, you have to have your legs under you, and you have to catch nine innings or close to nine innings," Redmond said. "With that being his throwing arm, you have to make sure he gets his arm in shape. You want him to build his arm strength up, so when he gets here, he is game ready."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter