The singer/actor, who was a member of 1990s band 911, had to hobble offstage at the Pomegranate Theatre in Chesterfield, England after suffering the injury as he portrayed the title character. He pressed on to complete the performance, and was subsequently admitted to a local hospital for treatment.
A source told The Sun, "He went straight to A&E (Accident & Emergency) after, where he was told he had badly torn his calf muscle and would be on crutches for four to six weeks."
Despite his injury, Brennan refused to rest and instead returned for a later performance, using his crutches to support him throughout the show, in which he co-stars with another former pop star, Atomic Kitten member Liz Mcclarnon, who was cast as Princess Jasmine.
The Aladdin pantomime runs until 3 January (17).