"But I had no idea what happened," the UFC fighter tells PEOPLE of watching Ballas lying on the ground in pain during DWTS camera-blocking rehearsal on Sunday.
Ballas had many fancy tricks planned for the routine they were to perform on Monday's show, but because they weren't warmed up, VanZant had planned on skipping them at the rehearsal.
"We weren't warmed up, so I wasn't going to do the tricks. But he said we had to because we didn't have a lot of time," says VanZant. "We'd done the first trick 100 times. But the first time we did Sunday, he hurts his back."
From that moment, things got "really stressful," says VanZant.
Alan Bersten – a former So You Think You Can Dance contestant and current member of the DWTS dance troupe – was called in for two hours or rehearsal just in case Ballas would not be able to participate Monday. But it wasn't until just hours before the live show that VanZant was informed that Ballas would be unable to perform.
Courtesy Dancing with the stars
If Ballas is unable to return for the rest of season 22, it is unclear if Bersten will become VanZant's permanent partner.
"I feel horrible that Mark is hurting, but stepping in for him has been a dream come true," says Bersten. "But at the end of the day, it's all about Paige. I'm just happy I was there to support her tonight."
Adds VanZant: "Alan really stepped up today. I'm so glad I had him."
Dancing with the Stars airs Mondays (8 p.m. ET) on ABC.