Manchester City defender Manuel Akanji will be missing for "eight to 10 weeks" after tearing his abductor muscle, said Pep Guardiola.
Akanji suffered the issue in the first half of City's 3-2 defeat to Real Madrid in the Champions League on Tuesday and was replaced at the break.
City went on to lose thanks to a Jude Bellingham winner in the 92nd minute, having led twice.
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Akanji is expected to spend the majority of the remainder of this season on the sidelines.
Guardiola said: "Surgery on Saturday. Eight to 10 weeks out. His abductor is broken.
"We wish the best recovery for Manu because the effort he has done this season has been unbelievable.
"Him and Nathan [Ake] have played in difficult conditions, not being in top [shape], to help the team in the emergency we had with a lack of players we had in the centre-back position.
"In the end, the body said, 'enough is enough'."
Guardiola was more positive with an update on Jack Grealish after he too limped off in the first-half of the Champions League knockout phase play-off first-leg defeat.
The attacking midfielder is a doubt to face Newcastle on Saturday, but his injury is less serious than Akanji's.
"It's not as tough as what Manu [Akanji] has done but I don't know if tomorrow he will be ready," Guardiola said. "I don't think so, but we will see, we will assess in the next hours."
Guardiola also had an update on deadline-day signing Nico Gonzalez after the midfielder was forced off early on his Man City debut against Leyton Orient last weekend and was an unused substitute against Madrid.
"It's not a difficult start, he was just injured. A difficult start is when you play bad. That was not the case for the 20 minutes he played [against Leyton Orient].
"But of course it was a setback from when he arrived. He had a little pain. For Madrid, he was not fully fit. Day by day he is getting better. Like I said with Jack [Grealish], we will assess."