"He injured his back at camera blocking rehearsal on Sunday," a source tells PEOPLE of the newly engaged pro dancer. "It's an old injury that he aggravated. It's up in the air if he'll be able to perform tonight."
Just two weeks into the competition, this is not the first injury of season 22 – deaf model Nyle DiMarco had to get stitches after an accident during the first week of rehearsal.
Ballas' fate in the competition will be decided by a doctor later today. But until he is cleared by the medical team, Ballas' partner, UFC fighter Paige VanZant, is rehearsing with Alan Bersten, a former So You Think You Can Dance contestant and current member of the DWTS dance troupe.
"Mark has been so supportive and just a lot of fun to work with," VanZant told PEOPLE after last Monday's DWTS premiere. "I don't know how I'd do this without him."
Dancing with the Stars airs Mondays (8 p.m. ET) on ABC.