De Bruyne attempted to play on, but slumped to the ground moments later and then walked off in the 86th minute.
"We want him here, but at the end of the day it's him, agents and his family that decide", Guardiola said.
The 27-year-old has already missed two months this season after damaging ligaments in his right knee during training in August.
Guardiola said the Premier League champions were eager to keep Diaz, but added the decision was in the hands of the Spain youth worldwide.
He is expected to miss Sunday's clash with Southampton, Wednesday's Champions League game with Shakhtar Donetsk and the visit of Manchester United on November 11.
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Diaz's goals against Fulham were his first for the club and his contract situation has sparked reported interest from Real Madrid. "He knew that in the beginning and he chose to stay". He understands the way we want to play.
"Yesterday he came back in many, many things".
Manchester City have confirmed the news after De Bruyne was substituted during the club's Carabao Cup victory over Fulham, when Cottagers defender Timothy Fosu-Mensah fell backwards onto the Belgian, who was sat on the pitch with his legs stretched out in front of him.
"I think today Kevin is back".
"Today again he was involved with everything offensively and defensively, so hopefully what happened in the last minutes is not serious".