"Have missed days of rehearsal this week," Macchio, 49, posted on his Twitter account Monday. "Had a ruptured cyst on the back of my knee causing acute pain. Hoping to be able to perform tonight!"
Macchio and partner Karina Smirnoff are one of five couples remaining in the competition. Each couple is expected to perform two routines Monday.
All season long, Macchio has dealt with the aches and pains associated with Dancing's long hours of training. This time he has Baker’s cyst, which is a buildup of fluid behind the knee.
"I'm an old man," the Karate Kid star joked last month to PEOPLE. "I have a pain management routine that involves a lot of Advil ... It’s some little injury every week. I take a lot of ibuprofen. Old bones, old muscles, young character."
But having gotten this far in the season, Macchio vows the show will go on. "Nothing's gonna ever keep me down," he Tweeted.
Later he added, "Will be rough around the edges w/o rehearse time. But performing with @Karina_Smirnoff for u guys is the hi-light of every week. Hangin in!"
Dancing with the Stars airs Monday at 8 p.m. on ABC.