Walker replaced substitute goalkeeper Claudio Bravo, who was sent off for a sliding tackle on Josip Ilicic outside the box, having replaced first-choice keeper Ederson at half-time.

Raheem Sterling had given the visitors a 1-0 lead in the first half of their group-stage game before Chelsea loannee Mario Pasalic equalised four minutes after the restart.

City striker Gabriel Jesus also missed a first-half penalty in a bizarre game at the San Siro.

A victory would have sent City through to the last 16 but they remain five points clear at the top of Group C despite failing to win for the first time in the group stages this season.

Things went to plan after seven minutes when Sterling coolly slotted into the bottom corner after a brilliant backheel flick from Jesus.

But the Brazilian forward's penalty miss and Pasalic's thumping header early in the second half rocked the boat - City were no longer in control and Atalanta were posing a threat.

With Ederson substituted at half-time for a suspected injury, there was nervousness at the back and Bravo's rash tackle meant an outfield player was forced to go in goal.

Up stepped Walker, after a six-minute delay while Bravo's red card was checked by the Video Assistant Referee, and his first action was to make a smart save from Ruslan Malinovskyi's free-kick.

Walker, only the third outfield player to go in goal during a Champions League match, actually made more saves than both of Man City's recognised keepers during the game.

Goalkeeper chaos

The moment Bravo came on, City looked nervous at the back.

The Chile international played with fire on several occasions, coming out of his box to make a diving header and taking his time with clearances while being pressed by Atalanta's forwards.

He conceded within four minutes of coming on - though he could do nothing about Pasalic's terrific header, which came at him with pace from an unmarked position in the box.

And when Bravo came charging out of his area sliding, bringing down Ilicic and consequently being shown a red card, it caused chaos for City, who had no other keepers on the bench to turn to.

Walker was given instructions on the sidelines while the big screen in the stadium showed 'VAR check' but it took six minutes for his substitution to be made.

He high-fived Riyad Mahrez, who was sacrificed on his behalf, before running straight over to the goalposts and organising the defence into a wall to prepare for the free-kick.

Malinovskyi, who had come on for Atalanta during the six-minute wait, hit it low and straight down the middle but Walker got his body behind it and gobbled up the rebound, to great cheers from the travelling City fans.

Dominance wasted in first half

Manchester City should have had the game wrapped up in the first half but instead, spent the final seven minutes of stoppage time keeping the ball in the corner to prevent Atalanta from having a shot at Walker.

In the first half, Man City had eight shots, including six inside the box and had Jesus scored his spot kick, they would have been 2-0 up after 43 minutes.

Jesus, who has missed three of his seven penalties in all competitions for City, had a chance early on too when he was played in by Kevin de Bruyne, but his first touch let him down.

And when asked whether Jesus' penalty miss effected the game, Guardiola told BT Sport: "Definitely. Football is emotion."

Sterling also came close - missing Mahrez's cross by inches at the back post before the ball was taken away from him as he was about to shoot from a few yards out.

They were ultimately punished for their lack of ruthlessness and sloppiness at the back - something Liverpool will hope to take advantage of when the two Premier League rivals go head-to-head in Sunday's game at Anfield.

Atalanta had lost their previous three group games and this was their first point in the Champions League this season.

'In the second half we suffered'

Manchester City boss Pep Guardiola on BT Sport: "In this competition you know you have your chances and moments and you have to take it. But with the problems we have, we made a good first half. First half, we were outstanding and second, we suffered. In the last 15 minutes we had the issue with the new keeper.

"The second half we didn't do exactly what we should do. It was few chances conceded against one of the teams who create more. It was a perfect result away and we need one more point to go though.

"When we land in Manchester we will think about the next game in the Premier League."

A first for Bravo - best of the stats

-Manchester City failed to win a Champions League group stage game they were winning at half-time for just the second time, also doing so against CSKA Moscow in October 2014 (2-2)
City have been shown more red cards in their 18 games in all competitions this season (3) than they were in 61 games last term (2)
-This was Guardiola's 600th game in charge of a top-flight club in all competitions (W440, D95, L65)
-Six of Sterling's 19 Champions League goals have been against Italian sides, more than he's scored against opponents from any other country in the competition.
-Bravo became the first substitute goalkeeper to be sent off in Champions League history
-Gabriel Jesus has missed three of his seven penalties taken in all competitions for Manchester City, with this his first failure in the Champions League

What's next?

Manchester City travel to Anfield for a crucial Premier League fixture against leaders Liverpool on Sunday (16:30 GMT), hoping to close their six-point gap at the top. City are back in European action on Tuesday, 26 November when they host Shakhtar Donetsk at Etihad Stadium.