Erik ten Hag wished his former Ajax striker Sebastien Haller well after Borussia Dortmund's new signing was forced to leave the club's pre-season training camp due to a testicular tumour.
Haller enjoyed a stellar season under Ten Hag's management of the Eredivisie giants in 2021-22, scoring 21 league goals and netting 11 times in eight Champions League games to earn a move to Bundesliga giants Dortmund.
But Haller stepped away from his new club's pre-season training camp in Switzerland on Monday after feeling unwell during a session, with subsequent tests revealing the tumour.
The Ivory Coast international told social media followers he was focused on his recovery on Tuesday as Dortmund pledged to offer an update on his condition when possible.
Haller began a fruitful relationship with Ten Hag when the Dutchman became Utrecht coach in 2015, the striker plundering 30 goals across two seasons under him before the duo were reunited at Ajax in 2020.
Speaking after his Manchester United side posted a 3-1 pre-season win over Crystal Palace, Ten Hag sent a message of support to Haller.
"It's horrible," he told MUTV. "I have a fantastic relationship with him.
"He's my friend, he was my striker for several years with Utrecht and Ajax, and when you hear such news it's devastating.
"My thoughts are with him, with his family, and I hope he will recover quickly."
Meanwhile, Haller's current coach Edin Terzic described the situation as "brutally difficult" as he joined Ten Hag in sending his best wishes to the striker.
"Of course, we were shocked to hear the news yesterday," Terzic told Dortmund's media channels. "I personally heard it about half an hour before we were preparing to leave for the game [a 3-1 friendly loss to Valencia].
"It was not easy for all of us to find the switch, we tried to do it as professionally as possible, but then we didn't talk about it with the team before the game, but informed the team after the game.
"Now I ask you to understand that it is brutally difficult for us. The diagnosis is still pending.
"Sebastien left us last night, went to Germany today for further examinations, and we are keeping our fingers crossed for Sebastien and his whole family that he will get well as soon as possible.
"First of all, it is about the human being Sebastien Haller getting well again.
"As for the sportive issues that come with this loss, we haven't thought about those yet because we are still awaiting the diagnosis. We don't even know yet how long he will be out."