Germany international Nicolai Mueller faces half a year on the sidelines after rupturing his cruciate knee ligament as he celebrated scoring in Hamburg's opening game of the season.
The 29-year-old was left red-faced after scoring the winner in the 1-0 home Bundesliga victory against Augsburg eight minutes into Saturday's match.
After netting, Mueller twisted away, repeatedly twirling in his goal celebration and leapt at the corner flag before tumbling to the floor in obvious pain.
He tried to play on, but had to be replaced after 15 minutes.
"That is certainly a downer, I have never experienced anything like it before," said Hamburg coach Markus Gisdol.
Hamburg's worst fears were confirmed on Sunday and the diagnosis means Mueller, who made two appearances for Germany on their 2013 tour of the USA, is out for at least six months.
"The diagnosis is a big shock for us, a severe blow to the start of the season and, of course, extremely bitter for Nicolai too," said director of sport Jens Todt.
Gisdol has not ruled out making a signing to cover Mueller's injury before the current transfer window closes at the end of August, especially as US international forward Bobby Wood struggled against Augsburg.
Having been dumped out of the German Cup by third-division Osnabruck in last weekend's first round matches, this is another blow for Hamburg, who have only narrowly avoided relegation in three of the last four seasons.