The 23-year-old has been stalling over signing a new contract, and Hummels believes that this is an indication that Kagawa is preparing for a move elsewhere.
"Shinji is a world class player, but honestly, I don't expect him to stay with us. Otherwise, he would have extended his contract by now," Hummels told Bild.
"However, I'm not scared because we're going to get Marco Reus and we won't be losing any quality."
Borussia Dortmund have the chance to complete a domestic double on Saturday when they face Bayern Munich in the DFB-Pokal final, and although Jurgen Klopp's men have overcome their rivals twice in the league this season, Hummels is expecting a different challenge this time around.
"They'll definitely take us seriously. They'll play differently in Berlin."
Borussia Dortmund and Bayern Munich kick off at 20.00CET on Saturday in the German capital.