"I don't know if it's nothing or something serious", Guardiola said of De Bruyne, who suffered a knee ligament injury in training in August.
Premier League leaders City host Southampton at 15:00 GMT on Sunday.
At Friday's press conference boss Pep Guardiola said: "Unfortunately he is injured but fortunately he will come back in four-six weeks", before adding: "He was so sad". Asked if he was anxious about the injury, Guardiola said, "I don't know right now". "We want him, but in the end, it doesn't depend on us". "We have a very good opinion of him". But his agent, and him, finally decides.
"Playing in the top teams is not easy for young players, but he (Brahim) showed his qualities again". We are delighted to and will do absolutely everything to keep him with us.
Brendan Rodgers confident Celtic can still qualify despite latest setback
Defender Dedryck Boyata, whom Rodgers described as "outstanding", is suspended for the trip to Edinburgh. Celtic are now third in their Europa League group, three points behind leaders Red Bull Salzburg.
"He knew that from the beginning and he chose to stay".
Diaz's first goal was fortunate, his shot taking a deflection off Fulham striker Aleksandar Mitrovic and flying past flat-footed goalkeeper Sergio Rico.
Man City are unbeaten in all 19 Premier League matches when Aymeric Laporte has played (W17 D2 L0) - he has played more games without losing than any other player in the history of the competition, ending on the winning side in all 10 home games. "It was the Kevin we know".
"It was a hard situation because of the pitch and the conditions and tough physicality but today again he was involved in the offensive and defensive side, in the pace, in the set pieces, the passes and assists".