Manchester City captain Vincent Kompany asked to be replaced at half-time in the 1-0 EFL Cup loss at Manchester United because he was “tired”.
The defender, 30, was making only his second start of the season after suffering a groin strain in September.
“He told us he is tired and was not ready to play the second half,” said City boss Pep Guardiola, whose side have not won in six matches.
“The important thing is he is not injured – that is a good step.”
Kompany, who has been troubled by calf and shin problems in recent seasons, has said he will be “honest” with the medical staff over his injuries.
The Belgium international played 25 matches for club and country last season, a fall from 38 a season earlier and 40 in 2013-14.
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Kompany was replaced by Aleksandar Kolarov at Old Trafford, with Juan Mata scoring nine minutes after half-time to secure United’s place in the quarter-finals, where they will meet West Ham.
United boss Jose Mourinho said his side’s 4-0 Premier League defeat at Chelsea on Sunday influenced his decision to make only four changes for the visit of City, who made nine from the weekend.
“It was a big defeat – numbers that the history of this club doesn’t deserve,” said Mourinho, who replaced Louis van Gaal in the summer.
“When I’m in a club my heart belongs to the fans and I felt deeply for them.
“Today the stadium was full of real support and it looks like the love people have for the club is bigger than bad results. We must give something back.”
Mourinho said this week that life in Manchester was “a bit of a disaster” as his family live in London and photographers prevent him leaving his city-centre hotel freely.
But after seeing his side win for the second time in five matches, he added: “The only thing I can say is I couldn’t be prouder than I am to be Man Utd manager.
“The only way I know is to fight – to work and fight.”