Hamburg player out for 7 months after rupturing knee ligament in goal celebration gone wrong (VIDEO)
Muller scored what turned out to be the winner in the ninth minute as Hamburg beat Bundesliga rivals Augsburg 1-0 on Saturday, but was substituted shortly afterwards after twisting his knee while performing a 'helicopter' celebration which involved several pirouettes.
The 29-year-old has been ruled out for seven months with the injury. Muller joined Hamburg in 2014, and has made two appearances for the German national team.
"The diagnosis has come as a huge shock to us," Hamburg sporting director Jens Todt said, ESPN FC reported.
"It's a serious blow to our season and obviously absolutely gutting for Nicolai.
"We wish him all the best in his recovery and will support him as best we can throughout it."
The player himself said: "I was very unfortunate. I slipped with my right leg after my usual helicopter celebration. My family stabilises me, but right now there are not many things which could cheer me up."
The club also tweeted its support for Muller.