Foden was handed his first European start of the season two days after he signed a new long-term contract, and he impressed while completing the 90 minutes.
City needed just a point to clinch top spot in Group F and, despite falling behind to Andrej Kramaric's goal, Leroy Sane's brace ensured they secured victory at the Etihad Stadium.
Foden had been restricted to starts in the EFL Cup prior to Tuesday, but Guardiola said he never had any fears that the player could perform at the highest level.
"In general, Phil was incredible," Guardiola told a news conference.
"I don't think I will ever have doubts about the guy. It's the way he fights with the ball – he is not skinny, he is really, really strong.
"He is a huge talent. England have a diamond.
"I know it is not easy to play in the big clubs. He has to compete in a good way with David Silva, Bernardo [Silva] and Kevin De Bruyne. But he is incredible in the training sessions every day.
"Against a German team it is so demanding and he played like a real man."
Some have called for Foden to be sent out on loan due to his lack of first-team opportunities at City. However, the midfielder insists he is just eager to impress when selected.
"I enjoyed it - to get 90 minutes under my belt," Foden told BT Sport.
"I'm trying to take [the] chance when I can, we got off to slow start but got the win and that's what matters.
"I've got my contract and I'm trying to settle, keep my head down and take my chance when it comes.
"I'm still young, learning every day and enjoying every minute of it."