Borussia Dortmund CEO Hans-Joachim Watzke has defended Bayern Munich centre-back Mats Hummels following his visit to his former side's dressing room ahead of Saturday's meeting at the Westfalenstadion.
Hummels, 27, left Dortmund to return to Bayern -- where he first started his career -- in the summer.
The Germany international was part of the Bayern side that faced Dortmund in the Bundesliga on Saturday, and prior to the match he went into his old club's dressing room to say hello to his ex-teammates.
The visit was later criticised by former Bayern player and Sky Deutschland pundit Lothar Matthaus, who said that "before the game, the concentration should be on the game and not the old friendship."
Hummels said he did not care about Matthaus' criticism as all he wanted to do was to "say hello to a few people," and Watzke has since leapt to his defence.
"I think it was fantastic Mats Hummels visited his old colleagues far before the match," Watzke said during Dortmund's annual shareholder meeting. "When I then hear that this is regarded as unprofessional, I can only say it's disgusting. It shows Mats' human qualities."
Dortmund saw off Bayern with a 1-0 win on Saturday, with Hummels booed throughout by the home supporters.