Pep Guardiola feels guilty about not using Phil Foden against Fulham but said Sergio Aguero's ankle injury meant he could not bring the teenager off the bench.
Guardiola touted Foden for an England call-up prior to Saturday's Premier League game at the Etihad Stadium, when Leroy Sane, David Silva and Raheem Sterling were on target in a 3-0 win.
But Foden, who has made one brief substitute appearance in the Premier League this season, remained an unused substitute despite City easing to a comfortable victory against the newly promoted side.
City host Lyon in their first Champions League group game on Wednesday and Guardiola indicated the match was a factor in his decision to bring off Aguero early in the second half.
"I feel guilty. I feel guilty. He deserves to play but doesn't play," Guardiola said of 18-year-old Foden.
"We had a problem with Sergio in the first half and didn't want to take a risk. I feel guilty because he deserves to play but he's going to play a lot of games, I feel it.
"Today the guys are so exhausted and now we start two games a week so he's going to play. I like him a lot and every time he plays he plays so well, but all I can say is he's going to play."
Guardiola clarified Aguero's injury is not thought to be serious, with the striker replaced by Gabriel Jesus nine minutes after the restart having set up Sterling for his goal.
"He jumped and fell down and felt something," Guardiola said. "He finished the first half and in the second he felt a bit uncomfortable but after 3-0 there was no need to risk it. We'll see how he recovers tomorrow."
Fulham conceded a host of clear chances with Guardiola claiming his side could have scored "five, six or seven" goals against Slavisa Jokanovic's men.
But the Serbian hit out against criticism of teenage left-back Ryan Sessegnon, who was given a difficult time by City's array of attacking talent.
"There are benefits [of playing] which are huge and he must be happy and satisfied," Jokanovic told reporters.
"Your opinion is he didn't play good enough, mine is different and at the end I think he played well, he was a little shy and I expect from his side he will be stronger in both sides. We can all have different opinions.
"Behind him is 100 games in the Championship and now it's time to play in these places. I can repeat, the benefit is huge and he's not a small kid.
"Unfortunately we weren't surprised today, if you analyse the game Ryan played good and it's good for him and for his development and for his experience."
