Ralf Rangnick has reportedly been rushed to hospital for emergency surgery after an initial operation led to complications.
Rangnick, 67, spent a difficult six months in interim charge of Man United between November 2021 and May 2022 after Ole Gunnar Solskjaer had been sacked.
The German has since been Austria manager and led the country to the last-16 stage at Euro 2024 when they topped a group that included France, the Netherlands and Poland.
However, last month the 67-year-old was forced to undergo a routine operation on his ankle amid general wear and tear, according to The Sun.