* Billionaire turns loan into club stake
* Stadium to get back its original name (Updates with more details, quotes)
By Karolos Grohmann
BERLIN, Jan 22 (Reuters) - Billionaire Klaus-Michael Kuehne has acquired a 7.5 percent stake at former European champions Hamburg SV after pumping in 18.75 million euros in investment and also bagged the stadium naming rights, the club said on Thursday.
Hamburg, struggling to avoid relegation in the past few seasons, has been dogged by financial problems and a string of expensive but unsuccessful transfers.
"I am very happy that we could successfully wrap up our talks about me becoming a shareholder," Kuehne said in a statement. "I hope I can contribute to turning Hamburg around."
Kuehne essentially turned part of his 25 million euro ($29.09 million) loan to Hamburg last summer that allowed them to sign Lewis Holtby, Swiss international Valon Behrami and U.S. international Julian Green among others, into a stake in the former German champions.
His investment is not only a much-needed shot in the arm for Hamburg, pushing back commitments for the indebted club, but a major hope for the long-term future of the only Bundesliga club to never have played in the second division since the top league's creation in 1963.
The billionaire will also give the stadium its old name 'Volksparkstadion' back from July 1, 2015 after acquiring the naming rights "initially" for four years.
"For me as a Hamburg fan it was a matter of the heart that the home of the club has again its traditional name," Kuehne said in a club statement.
"I wish that the club can enjoy success at the Volksparkstadion like in the past.
Hamburg's stadium, currently called the Imtech Arena, was known as the Volksparkstadion until 2001.
"We are continuing our financial consolidation with this move and have reached an important milestone," Karl Gernandt, Hamburg football board chairman, told reporters.
"We will continue working so that Hamburg can win back its ability to be competitive on all levels in the Bundesliga."
Hamburg are 14th in the league, two points above last-placed Freiburg. The Bundesliga resumes on Jan. 30.
($1 = 0.8595 Euros) (Reporting by Karolos Grohmann,; Editing by Amlan Chakraborty and Pritha Sarkar)