The 21-year-old England midfielder was introduced in the 68th minute, and less than 90 seconds later Manchester City were 1-0 in front and on their way to securing a perfect start to a 10-day period that might end up defining their season.
It's Liverpool on Sunday in the Premier League, followed by the return leg in Madrid next Tuesday, then the FA Cup semifinal with Liverpool at Wembley on Saturday. In just 20 minutes here, Foden made his case to start them all.
He was the difference in the Champions League quarterfinal first leg so much so that when the final whistle went, the goal scorer, Kevin De Bruyne, went straight over to offer a hug and a handshake. Had Foden stayed on the bench, City might not have found a way through Simeone's rigid game plan and would be heading to Spain next week with the tie on a knife edge.
"A difficult game against a tough opponent, it's so difficult to find gaps," Guardiola said afterward. "They defend so well, so compact and so deep and we need the talent like Phil has shown in small spaces. They can punish you on the counterattack because they are top, top players. Phil has a special quality. His reception is always forward and he has the composure to make an assist for Kevin.
"It's a good result, unfortunately at the end we had one or two more chances, but 1-0 to go to Madrid is good. Honestly, I was not expecting to win 3- or 4-nil."
Games against Atletico are, by Simeone's design, often decided by fine margins, but Foden is good enough to thread a pinpoint pass through the tightest of spaces.
"We tried to keep it tight, use the counterattack. They didn't get a shot on target in the first half, but we couldn't hurt them either," Simeone said after the match. "In the second half we had some more dangerous counterattacks, then they scored. A very tactical game, both [coaches] looking with what he has to try to go through."
Guardiola and Simeone wore the same long black coat on the touchline to guard against the Manchester rain, but their footballing styles could not have been more different. For long periods Atletico's red and white shirts were entrenched in two lines in front of Jan Oblak's goal, while Guardiola spent most of the first half telling John Stones -- his centre-back -- to push higher up the pitch. After 20 minutes, City had recorded 80% possession, although Guardiola seemed well aware of the threat lurking within the Spanish champions' low block.
Anytime Atletico sprung out the traps, even if it was from the edge of their own penalty area, Guardiola crouched down in his technical area bracing himself for the worst. Twice in the opening five minutes, Atletico tried to play into the space behind Nathan Ake, deputising at left-back in the reshuffle that saw Joao Cancelo moved to right-back to compensate for Kyle Walker's absence through suspension. But Atleti's threat going forward was limited all night.
Speaking at his news conference on Monday, Guardiola appeared to bristle at the notion that Atletico are a team primarily focused on defending. However, there was little in the first half to suggest otherwise. Simeone's side went in at half-time without having a shot at Ederson or even a corner. It might not be football to drool over, but Atletico have turned defending into an art form, and with two thirds of the game gone, City's best chance was an Aymeric Laporte header from a corner.
"They are very good," added Guardiola. "They have been many years together. We created very few things. They are very good at many things, and they have the patience and time to defend for long periods of time. It is not easy to face a team with a lot of experience in this tournament. We will go there to score and try and win again."
Appreciate Simeone's style or not, very few do manage to do that at the Etihad these days. City had failed to score in only one of their 28 previous Champions League home games under Guardiola, but it was only when Foden came on -- introduced alongside Jack Grealish and Gabriel Jesus -- that they looked likely to make it one in 29. Almost immediately he picked up the ball in a tight space on the edge of the box, drew five Atletico defenders toward him and lifted his head to slip a pass through to De Bruyne. It was one of two outstanding moments of quality in the game, the other coming from Foden again when he danced his way down the byline to create another chance for De Bruyne.
"We thought he'd start, he's very dynamic, very quick, powerful in finishing off moves," Simeone said about Foden. "He came on in the second half. Any of the three that came on have good characteristics. I like to watch Manchester play, the patience they have to play."
The Atleti striker, Joao Felix, is regularly talked about as a future Ballon d'Or winner, but Foden -- a year younger -- deserves to be mentioned in the same conversation. Carry on influencing the biggest games and it won't be long before he's at the centre of it. He should get another chance against Liverpool on Sunday as City's season reaches a critical stage. One down, three to go.