Biggest thing was Pep pulled a surprise by playing Foden as a false 9 who dropped onto Jorginho whenever City were off the ball - this left them 3v2 everywhere else in midfield, so we had to go for long balls as they could press us into oblivion. Other issue was that us playing a 3-5-2 meant that their fullbacks had time and space - Walker wasn't a problem, but Cancelo very much was. Although both our strikers were isolated 1v1, neither could do much. Lukaku was reasonable in terms of hold up play but Timo was awful - though the latter did create space with a couple runs in behind.
Personally I thought Chelsea should have gone back to 3-4-3 sooner - we certainly looked better once Havertz was on, but at the same time City were content to sit back and hit us on the counter (and to be fair were closer to making it 2-0 than we were to drawing level).
Mount not being fit was a killer. Neither Lukaku nor Timo were effective when dropping into midfield to link defense and attack and that's Mount's bread and butter (and again, Havertz did a reasonable facsimile of this once he was on, but for me he's better further forward).