Mainz will be without Danish forward Emil Berggreen for up to six months after he tore cruciate knee ligaments in training.
Berggreen, 23, joined Mainz in January only to suffer a serious knee injury in one of his first training sessions.
He was only just back in full training when the latest injury occurred, and he will now have to wait at least until 2017 before making his Mainz debut.
"We're incredibly sorry -- he will get all the support he needs from us," Mainz's director of sport Rouven Schroder said on the club's Facebook account, where he also appealed to fans to send their messages of support for the Dane.
Berggreen scored 10 goals in 26 appearances for Eintracht Braunschweig in Germany's second division before making the switch to Mainz in the winter, but is yet to make his first appearance for the club.