Hamburg have confirmed Nicolai Muller suffered a torn anterior cruciate ligament celebrating his goal against Augsburg in the first round of the Bundesliga season on Saturday.
Muller netted what proved to be the only goal of a 1-0 win in the eighth minute of the game at Volksparkstadion but had to be substituted after falling awkwardly while celebrating his strike.
The 29-year-old underwent scans on Sunday to determine the full extent of the injury, which is likely to leave him out of action for seven months.
"The diagnosis comes as a huge shock to us, it's a big blow at the start of the season and obviously is really tough on Nicolai," Hamburg's sporting director Jens Todt was quoted as saying by the Bundesliga website.
Terrible news: Nicolai #Müller has ruptured the anterior cruciate ligament in his right knee and will be out for approximately 7 months. pic.twitter.com/PVm0q2gbj0 — Hamburger SV (@HSV_English) August 20, 2017
"We wish him all the best for his recovery and, of course, will support him as best we can."
Hamburg's head coach Markus Gisdol said: "It's bad news. I've never seen anything like it."