When the Chicago Fire struggled with a rash of injuries at the start of this season, there was no doubt which injured player they missed most.

On Sunday morning, center back Arne Friedrich made official what had grown clear as the season progressed: He would have to retire from professional soccer, making the announcement on Twitter from his native Germany, where he had gone for a second time this season in search of effective treatment for leg and back pain.

"This was a very difficult decision," Friedrich said in a team press release. "After discussing it with the medical staff and my family, I've decided that it is the right time for me to retire from professional soccer. I was very excited to re-sign with the Fire following the 2012 season but unfortunately my injuries prevent me from returning to the game that I love.

"I have been truly blessed to represent my country at the highest level and play for world-class clubs. I would like to thank all of my teammates, coaches, the fans and my family for supporting me throughout my career."

Friedrich, who also played for Hertha Berlin and Wolfsburg in the German Bundeliga before joining the Fire, made 82 appearances for Germany's national team. He joined the Fire at the start of the 2012 season and was a key component of the Fire's return to the playoffs, often seeming to will his teammates to victory with his determined work on the back line or by going forward to launch an attack on the opponent's goal himself.

He brought the classy play of a World Cup veteran to Toyota Park without bringing an attitude, mixing easily with his Chicago teammates, fans and the media.

And even at the very end of his career, he did what was best for the Fire.

By retiring now, Friedrich, the Fire's second-highest paid player at $367,500 this season, according to the MLS Players Union, frees up salary-cap space for a club that is close to the $2.95 million cap.

With the secondary transfer window to open July 9, Friedrich's departure makes it easier for the Fire to go find the attacking player it needs -- possibly from Mexico, possibly from Europe.

But the Fire will still miss Friedrich.

Follow Orrin on Twitter @orrin_schwarz