Foden recorded his fourth goal in three games in all competitions to help City to a 2-0 victory over Newcastle United that kept the pressure on Premier League leaders Arsenal.
The winger had been in and out of the side due to an ankle injury suffered against Manchester United in January and Guardiola said the 22-year-old, facing the first adversity of his young career, lacked confidence on his return.
"(I told him) it was important to accept it, the struggle with his ankle, accept it," Guardiola told a news conference.
"That's the most important thing. Don't blame the opponent, don't blame the manager or the club or the mates or whatever. Accept: I can do better. Come back to your principles and step by step you will be back."
Foden showed brilliant control muscling through the Newcastle defence to give City the lead in the 15th minute on Saturday - his fourth goal in a week after netting one against Bournemouth and two against Bristol City in the FA Cup.
His diagonal shot glanced off the foot of Newcastle defender Sven Botman and into the net.
Guardiola said Foden has been "one of the most important players up front" in recent weeks, creating chances for players like striker Erling Haaland when he wasn't scoring himself.
"What happened with Phil was completely, completely normal," the Spaniard added. "I said, the bad moments, these too shall pass. It's always like that."
