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Mats Hummels has seen his stock rise at Dortmund

The Borussia Dortmund defender has been attracting interest from the Premier League table-toppers as they look to firm up their summer shopping list.

United have conceded many soft goals this season, and Sir Alex Ferguson is keen to provide some stability to his defence.

And with Rio Ferdinand's future at Old Trafford in the balance, Hummels is considered a suitable replacement.

However, Barcelona are keen to table a bid, perhaps as an alternative to Alex Song, who has been viewed in polls in Spanish papers as one of the worst summer signings in La Liga along with Real Madrid's Luka Modric. Mats Hummels has caught Sir Alex Ferguson's eye

Sport.es claims Hummels has been linked with a move to Barcelona for the past two seasons

Spanish paper Sport.es claims Hummels has been linked with a move to Barcelona for the past two seasons.

They also suggest that Tito Vilanova is keen on the player and wants to table a bid this summer.

Hummels has already racked up 29 appearances for reigning Bundesliga champions Dortmund this season.

Hummels joined Dortmund from rivals Bayern Munich in 2008.