Bayern Munich striker Robert Lewandowski says it will not be easy for Mats Hummels to face his next club in Saturday's DFB-Pokal final.
The Bundesliga champions confirmed this month that the Borussia Dortmund captain will rejoin his boyhood club in July, after he agreed a five-year deal.
Hummels' final game for Dortmund comes in this weekend's showdown with Bayern and the 27-year-old has already promised supporters that he will "give everything" to secure the trophy.
But Lewandowski, who swapped Signal Iduna Park for the Allianz Arena in 2014, has warned that his former team-mate could find the occasion difficult.
"I know this situation. Even if you are mentally strong, it's still not easy," he said, as quoted by TZ.
"He'll give 100 per cent effort, that's for sure. I know Mats, he wants to show that he's still a BVB player and a professional, but it's not that easy for BVB."
Team-mate Thomas Muller expects an arduous battle against a Dortmund side who have fallen at the final hurdle to Bayern and Wolfsburg in the last two seasons.
"No matter what's come before, you're never given anything in a final," said Muller.
"They'll wait for us with a knife between their teeth. It'll be a battle of attrition."