Manchester City jumped out to a 3-0 lead over struggling Chelsea on three goals in the first 34 minutes. Steffen did not have much to do, conceding his only goal to Callum Hudson-Odoi in stoppage time.
"The result is incredible for us, especially the way that we played," Manchester City manager Pep Guardiola said. "Chelsea are one of the strongest teams, but we had a plan and tried to play how we've done in the past and it worked, the players were fantastic.
The start was the fifth for Steffen in 2020-21 after spending last season on loan to Fortuna Duesseldorf. He won three starts in the Carabao Cup and one in the UEFA Champions League against Marseille.
He will likely start on Wednesday when Manchester City faces Manchester United in the Carabao Cup semifinals.
Manchester City is one of several English teams hit by a COVID-19 outbreak. Its game against Everton last Monday at Goodison Park was postponed hours before kickoff.