Hummels played under Klopp during their time together at Borussia Dortmund.
However, while the Germany international has helped Bayern Munich to the top of the Bundesliga table, Klopp's season at Liverpool has imploded.
The Reds' title hopes are all but over with Liverpool 13 points behind league leaders Chelsea.
Klopp's men have endured a nightmare start to 2017, winning just one match (a 1-0 win against Plymouth) since the turn of the year.
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Thu, February 9, 2017
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Mats Hummels has admitted he regularly watches Liverpool
Liverpool are stuck in a rut under Jurgen Klopp
And Hummels has admitted he regularly watches Liverpool – and feels sorry for Klopp given the Reds' recent plight.
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“I do a lot," he told Goal when asked if he followed their matches.
“I watch the games if I have the time – nearly every one.
“Of course it’s a pity for me that he is having such a bad period at the moment. They were eliminated from the FA Cup and League Cup.
Mats Hummels played under Jurgen Klopp at Liverpool
“I’m supporting him and I wish him success. They are having a hard time at the moment.”
Hummels won two Bundesliga titles under Klopp in 2011 and 2012.
The 28-year-old was also part of the Dortmund side that lost to Liverpool in the Europa League last season.