In March 2012, he moved on loan to League One side Tranmere Rovers, in a move later extended to run until the end of the season. He made his professional debut on 17 March 2012 in a 1–1 draw at Sheffield United, and scored in his second appearance, a 2–0 win at Rochdale. In total, he scored five times in ten appearances.
Ahead of the 2012–13 season the striker renewed returned for a second loan period with Tranmere Rovers; a move which lasted until January 2013. He scored his first goals of his return with a hat-trick against Colchester United in a 4–0 win on 1 September 2012 that put Tranmere top of the league. Cassidy was then named the League One Player of the Month for September 2012, after scoring seven league goals during the month. He returned to Wolves after his loan spell ended in January 2013 having scored 11 goals in 26 appearances.