Sources close to the situation tell TMZ ... the accident occurred when Frank was at Coachella rehearsing his set. He wasn't onstage, though, we're told the mishap involved bikes used to roam around the Empire Polo Club grounds .. and it was serious enough for doctors to advise Frank's production be changed.

One of the major changes involved the use of an ice rink, which was built for Frank's set. Another source with direct knowledge tells us ... a casting director reached out to a bunch of L.A.-based hockey teams in March for an unspecified paid performance. Once the skaters got to the audition, they saw Frank in attendance.

We're told it became clear during the audition that it was for his Coachella set -- and over 100 hockey players were cast. They rehearsed on an elevated ice rink at Paramount Studios, and even got bused out to Coachella for a dress rehearsal the week before Frank's show.

The cast arrived at Coachella on Sunday, but were told the skating routine was scrapped. Instead, they joined Frank on stage as background dancers -- it was more walking than dancing, TBH -- and a source tells us they all got hooked up with custom Prada.

Questions surrounding why Frank arrived at his set late and how production will look for his Weekend 2 set still linger ... but we hope he's a fast healer.