Dortmund secured their place in the last 16 of the Europa League on Thursday night with a late goal from Schmelzer in their 1-1 draw against Atalanta. Hummels, who played alongside Schmelzer while at Dortmund, sent a tweet out after his fellow countryman scored.
The tweet was met with anger by the Bayern fans, with most of them questioning the centre-backs loyalty to the club.
Despite the tweets that were sent his way, Hummels responded in the most professional way possible and explained that he should be allowed to celebrate a goal that was scored by a dear friend and a former teammate who he played with for many years.
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Dortmund were nowhere near good enough on the night against a strong Atalanta side, and with Red Bull Salzburg as their opponents for the round of 16 they will need to up their game if they are to finish the season with some silverware.
Up next for die Schwarzgelben is a home fixture against Augsburg on Monday night, a game in which fans are set to boycott. The fans have stated that a Monday night fixture is not acceptable and by boycotting the match the fans will hope to send a message to the league that they must stick to the regular schedule or else the stadiums will become empty.