Manchester United have fallen behind in the race to sign defender Mats Hummels from Borussia Dortmund with Barcelona favourites for his signature. According to radio station Cadena Ser, Spanish champions Barcelona have held talks with the German defender about a summer move.googletag.cmd.push(function() { googletag.display(“div-gpt-ad-1389116196215-0”); });Hummels has been on United boss David Moyes’ list of targets for some time after some impressive displays in the Bundesliga. The 25-year-old German has only just returned to the Dortmund team after missing a chunk of this season through injury. Dortmund have underachieved this season, falling further behind leaders Bayern Munich and allowing Bayer Leverkusen to move ahead of them into second. The Westfalenstadion club’s form is thought to be a point of frustration for Hummels who is looking for a summer exit alongside teammate Robert Lewandowski. The Polish striker will sign for Bayern in the summer after signing a five-year deal in January. He will be the second Dortmund player to move to Bavaria in two seasons after Mario Gotze signed for Bayern last season.