Boyband singer and guitarist Fletcher's clash with English rugby union player Jackson Wray left him needing medical assistance.
His injury, which has affected his ability to play the guitar, could delay the band's spring tour.
Fletcher told The Mirror: "A guy from the rugby team knocked my shoulder a bit and it is partially dislocated.
"To be honest, he hardly touched me - which goes to show how massive and strong he is, or how embarrassingly puny I am. Probably a combination of the two."
The 26-year-old added: "I'll be fine... I just can't move properly at the moment!"
This spring, the 'Shine A Light' group are scheduled to play a 26-date UK tour, starting in Sheffield on March 18.
> McFly interview: 'We can't imagine splitting up'
Watch McFly's 'Shine A Light' music video below: