Manchester United are set to miss out on top summer transfer target Thomas Vermaelen after Arsenal agreed a £15 million bid from Barcelona for the centre-back, according to the BBC. Vermaelen had been at the centre of a two-way tussle between United and Barca but yesterday Gunners manager Arsene Wenger admitted he would prefer to sell the player abroad rather than to a domestic rival. The 28-year-old Belgian international is now set to discuss personal terms with the Spanish giants and is expected to undergo a medical at the Nou Camp later this week. The news will come as a crushing blow to United manager Louis van Gaal, who is in the market for a new centre-half following the summer departures of Rio Ferdinand and club-captain Nemanja Vidic. This isn't the first time van Gaal has been frustrated in the transfer market during his pursuit of a new centre-half. Borussia Dortmund manager Jurgen Klopp recently rubbished speculation linking Germany World Cup winner Mats Hummels with a move to Old Trafford while former target Ezequiel Garay has joined Zenit St Petersburg. United endured a disappointing transfer window 12 months ago under former manager David Moyes and missed out on a number of top targets - including Cesc Fabregas - before signing Marouane Fellaini on deadline day. Supporters will be hoping their club aren't set for a repeat of those failings this summer.