* New FDP head Philipp Roesler also to be deputy chancellor
* Unpopular Westerwelle remains as foreign minister for now
(Adds details, background)
By Annika Breidthardt and Thorsten Severin
BERLIN, April 5 (Reuters) - Chancellor Angela Merkel's Free Democrat (FDP) coalition partner picked her 38-year-old health minister as its next leader and deputy chancellor of Germany on Tuesday after her foreign minister quit the party post.
Philipp Roesler is to be formally elected at an FDP party congress in May, succeeding the unpopular Guido Westerwelle, who gave up the leadership on Sunday after state election defeats and poor polls brought a long-simmering party revolt to a head.
Westerwelle, who also quit his job as deputy chancellor, has said he wants to remain foreign minister but some have voiced doubts he will be able to cling onto that top cabinet post.
He is widely blamed for the FDP's plunge in support to about 5 percent from a record 14.6 percent in the 2009 federal election, which in turn has damaged Merkel's second term in office. Polls show he is the least liked German politician.
"This is about winning back credibility for the FDP," said Roesler as he announced his selection by the party leadership.
Roesler, adopted as a baby in a Vietnamese orphanage by a German couple, and trained as a doctor, ranked 15th out of 20 in a Spiegel magazine popularity poll of politicians.
That made him the second most-popular Free Democrat after the justice minister, and put him four places ahead of Westerwelle in 19th place and the FDP economy minister, Rainer Bruederle, who came last.
Critics have said Westerwelle, 49, has been arrogant, lacks a common touch and failed to convince as foreign minister.
STATE ELECTIONS LAST STRAW
Roesler must prove he is up to the job of reasserting the FDP's identity beyond just calling for tax cuts -- which seemed to make the FDP a single-issue party under Westerwelle, who was unable to deliver them because of the economic downturn.
Germany's European partners will also be watching the tone that Roesler sets on euro zone policy, after attempts by the FDP to strengthen its appeal by taking a tough stance on bailouts.
This has not helped the party in state elections this year. It failed to gain a seat in two of them and just scraped into the conservative stronghold of Baden-Wuerttemberg last month.
The FDP's poor performance in this election contributed to Merkel's Christian Democrats losing the state for the first time in 58 years, undermining her leadership of the coalition.
This humiliation, in which the conservatives' pro-nuclear stance played a big role, fuelled a revolt against Westerwelle. A prominent critic within the party has called Westerwelle a "millstone round the neck" of the FDP.
When Gerhard Schroeder's Social Democrat/Greens coalition lost key states in 2005 in similar fashion, he called a snap election, and lost. But Merkel is not expected to do the same.
She may be saved from having to reshuffle her cabinet again just weeks after the popular defence minister, Karl-Theodor zu Guttenberg, resigned in a plagiarism scandal. Predictions that the FDP shake-up could cost Westerwelle and 65-year-old Bruederle their cabinet posts did not materialise immediately as Roesler, initially seen as keen for the economy ministry, will remain health minister for now.
"There is no need to change things," Roesler said.
(Editing by Stephen Brown and Mark Heinrich)