Manchester United have confirmed that Noussair Mazraoui will be unavailable for a few weeks, having undergone minor surgery after experiencing heart palpitations.
The 26-year-old defender withdrew from last Sunday's 0-0 draw at Aston Villa at half-time before pulling out of international duty with Morocco.
He is now set to spend a short spell on the sidelines after a successful precautionary corrective procedure.
United have described it as a relatively common condition, and Mazraoui is expected to make a full recovery in the coming weeks.
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Harry Maguire was never going to re-emerge from the tunnel for the second half at Villa Park after pulling up with a leg problem right before the break.
But Mazraoui was a surprise absentee after seemingly last the first 45 minutes unscathed on the right side of United's defence.
Ten Hag replaced him with Victor Lindelof, though, and later remained coy on his £17million summer signing's condition.
"We had to take them off at half-time. Tomorrow, we have to see what is the diagnosis on it," the Dutchman explained in his post-match press conference.
He added to MUTV: "We have to assess this. I can't tell you now what the issue is.
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"I know what the issue is but [I cannot say now] how good or bad it is. We have to see assess this tomorrow [Monday] and then we will bring you news."
Mazraoui has featured in all 10 of United's games this season since arriving, starting in all of them except the 7-0 Carabao Cup win against Barnsley, in which he played half an hour.
Luke Shaw and Tyrell Malacia's continued injury absence has left the Moroccan and Diogo Dalot as Ten Hag's only senior full-back options.
Dalot has been operating on the left at the start of the campaign, with Mazraoui making the right-back position his own with some encouraging displays.
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The fact that the former Bayern Munich and Ajax star's heart issues aren't considered serious is a major positive.
But Mazraoui's upcoming absence serves as a major blow for Ten Hag in a period where he desperately needs results to go his way.
United host Brentford at Old Trafford in the Premier League on October 19 - the start of a chaotic seven-game run within a three-week period.
Trips to Fenerbahce and West Ham United follow, before four successive home fixtures against Leicester City twice (Carabao Cup and Premier League), Chelsea and PAOK.
