Manchester United have put in an offer of £21.5m for Borussia Dortmund defender Mats Hummels, according to reports.

The Red Devils are understood to be long-term admirers of the Germany international, who is near the top of their transfer wishlist this summer.

Louis van Gaal is thought to see the 26-year-old as the ideal man to strengthen a defence that has often been depleted by injuries this season.

Hummels has been heavily linked with a move away from Dortmund this summer, with his contract due to expire in 2017 and the club having endured a thoroughly disappointing season.

Manager Jurgen Klopp, whose impending departure could also have an impact on Hummels's future, last week admitted that the World Cup-winning centre-back was considering a move to Old Trafford.

That switch has now moved one step closer to fruition, according to Bild, with United lodging a formal offer for his services.

Hummels has made 240 appearances for Dortmund over his eight-year career at the Westfalenstadion, scoring 21 goals.