City also had a second academy product, Brahim Diaz, on the field at the end of the game as Guardiola won his 50th game in charge of the club.
But after Raheem Sterling's late strike ensured City stayed perfect in the group, Guardiola insisted Foden and Diaz will only play if they continue to prove themselves.
"Today is a special day for all of the club," Guardiola told BT Sport. "The academy have worked so hard for the last couple of years and we had Phil Foden and Brahim Diaz on in the end.
"They feel what it means to play in front of these fans. It depends on Phil now. He knows that the club, his team-mates and staff is there to support.
"He knows the club, he has to be calm, work every day. He knows that we trust him."
Having confirmed top spot in Group F, and with matchday six's game against Shakhtar Donetsk coming immediately before a derby Premier League clash with Manchester United, Guardiola was asked if he will play young talent like Foden and Diaz again.
"We respect the competition, we respect Shakhtar and we respect Napoli," Guardiola said.
"We have a lot of games, we will see what team we can play in the final game. Who knows what can happen."
Of City's battling performing in a tight match against a stubborn Feyenoord side, Guardiola added: "It was so difficult, so complicated to find the space. Sometimes it happens.
"The second half was much, much better. We had chances and we were able to win and we are top of the group. We are happy."