~Snip~
They are not going to lose that game unless Guardiola will heavily rotate out his best players in order to make sure that none of them gets injured before the most important games of the season against Real Madrid. But if they lose that game for some reason, it would most likely not make things much worse. I think it would raise the pressure tremendously to get something done in the Champions League. But anyway yes it would be bad morally, not really anything positive about losing.
Winning the FA Cup will be a motivation to boost the players' confidence after being crushed by Arsenal in the EPL. I think Guardiola will not take any risks in the FA Cup in order to maintain the fitness of his mainstay players against Real Madrid in the UCL play-off round.
The game is very important for Man City because the result can determine their fate in the UCL, maybe Guardiola will rest some of his mainstay players in the FA Cup to maintain the integrity of the squad in the UCL.