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.
RESULTS
PSG 1 – 0 Brujas
Real Madrid 6 – 0 Galatasaray
Bayern 2 – 0 Olympiacos
Red Star 0 – 4 Tottenham
D. Zagreb 3 – 3 Shakhtar
Atalanta 1 – 1 M. City
Lokomotiv 1 – 2 Juventus
Leverkusen 2 – 1 Atlético
Liverpool 2 – 1 Genk
Napoli 1 – 1 Salzburg
Barcelona 0 – 0 Slavia P.
B. Dortmund 3 – 2 Inter
Zenit 0 – 2 RB Leipzig
Lyon 3 – 1 Benfica
Chelsea 4 – 4 Ajax
Valencia 4 – 1 Lille
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