Manchester City's squad was indeed incomplete when they lost to Arsenal, but should that be the main reason why Manchester City lost and failed to score? I don't agree if the absence of Rodri and De Bruyne is the only cause of Manchester City's defeat, in fact they still have many other quality players such as Bernardo Silva, Phil Foden, Mateo Kovačić, and many others in the main line-up.
Manchester City's main problem in that match was that they failed to create many chances compared to Arsenal. The match statistics are almost the same, but Arsenal has more chances to score than Manchester City.
Having an incomplete squad is not an excuse for losing against
Arsenal cause City is a team that got good substitutions that could make the starting 11 of some EPL teams moreover Arsenal were also without there star player Saka and didn't even start Martinelli who helped them secure the victory, if they had done better with their performance like they did in the first 1st and even try to create better chances for their star player Haaland I believe they would had been leading in the 1st half.
If you compare both teams you'll notice that City got more quality players that played the game than Arsenal, if only Pep Guardiola didn't sale Cole Palmer to Chelsea maybe he would had helped to create chance and help them get a draw or even win the game.