Kovacic suffered a muscle injury in training which saw him sit out Tuesday's Champions League clash with Porto, which saw the Blues reach the semi-finals with a 2-1 aggregate win.
Speaking after the game, Tuchel gave an update on the 28-year-old.
"It was very sad to see him [on Monday] after 20 minutes in training, to have this little muscle injury from more or less out of nothing," said the Chelsea boss.
"We played in the boxes for the second day after training, just to have a little sweat and he felt his hamstring.
"So I think, for sure, for the game against City he is out, it is a big loss."
Tuchel added: "I was very sad for him because we don't have so many experienced players that have played Champions League many times.
"If you see him when the anthem is playing and he stands there, you see him relaxed and at the same time he is a hard fight, everybody is happy to have him around. So we also did it for him today."
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