Sarri decided to field Eden Hazard as a lone striker against the Premier League champions, with Olivier Giroud on the bench.
Morata has scored five goals thus far this season in the top-flight, but he has struggled for form recently, drawing blanks in each of his last four appearances.
He has struggled with the opposition's offside traps, too, particularly in his side's 3-1 loss to Tottenham.
Sarri, though, maintains that he always planned to use Hazard as a sole striker, thus explaining Morata's absence.
"It’s tactical because with the side to start with Eden in another position so I think it’s useless to have two strikers on the bench," he told BT Sport.
City are also without a recognised striker up front, with Sergio Aguero ruled out and Gabriel Jesus on the bench.
Raheem Sterling has thus been tasked with playing as a false nine, and Sarri expects the two teams to go at one another with great pace.
"I don’t know (why City chose not to select a striker) because in Manchester City maybe because they have out [Sergio] Aguero," Sarri added.
"Today maybe for the first time in Premier League there is the opportunity to have more spaces.
"I prefer to play faster football."
Pep Guardiola, who has overseen an unbeaten start to the season, has also explained his selection of the England international ahead of his Brazilian team-mate
He said: "Well we know the movement, defensive movements from Chelsea.
"We want to find some spaces with Raheem, that’s the reason why."
City dropped to second ahead of kick-off after Liverpool thumped Bournemouth 4-0 earlier in the day.
The champions have won 13 games and drawn two from their opening 15 encounters of the season.