Bayern Munich assistant coach Hermann Gerland claims it was a big mistake to sell defender Mats Hummels to Borussia Dortmund.

Gerland coached the youngster for two years in Bayern's reserve team before he was initially loaned out to the Westphalians.

He subsequently made the move permanent for 4.5million euros in 2009 and Gerland says his club now regrets that as they prepare to welcome him back to the Allianz Arena as an opponent this weekend.

"I always knew that Mats was going to be a star," Gerland told the Bild newspaper.

"We should never have been allowed to let him leave."

Hummels has claimed a regular place in the Dortmund team this season and has emerged as a possible candidate for the national team at the World Cup this summer.

"He is not only our best man-marker, but he has a irreproachable character," added Gerland.

"At the recent meeting of coaches, I spoke to (Germany coach) Jogi Low and also (Germany assistant coach) Hansi Flick about Mats' strengths.

"Maybe he will still make it (for the World Cup)."

The 21-year-old has the unenviable task of stopping Bayern's in-form attack this weekend as Dortmund look to end their 11-game winning streak.