For me failing is not just if he wont get top4 place. That would be disaster clearly.
Even 4th place is some kind of disaster that would mean Wenger or AVB can make their team finish above us while we have lot better squad.
3rd place can be easily the reality if Moyes wont have problems adapting to United while Mourinho will be very good what I expect anyway.
But under Mancini last 3 season we were 3rd on same points as Chelsea, so we were not really worse than the second placed team, then we finished first and last time we finished second with a 12 points gap to United.
I mean anything more than 12 points gap behind the 1st placed team would be serious dissapointment for me. Even 10-12 points...
If we finish 2nd or 3rd but keep it max. 7-8 points far from the champions that's OK as long as we show something great in the CL. (1/4 finals or even 1/2 finals depending on the opponents in later rounds.)
With the chance we get another group of death CL will be very hard and it could be another big fiasco in group stage.
Pellegrini came in to change that, but if we get Bayern/Real/Barca and PSG or Juve in the group we will have about 50% chance of failing once again and that is huge.
With an another bad CL campaign I expect PL title.
With only 2nd or 3rd place in PL I expect really good CL run.
4th place or worse in PL and CL group fiasco would be a joke, probably I could do that with this squad.
Pellegrini has a lot to prove, he said himself he knows City have the best squad in the PL in his interview, hopefully he wont make it finish 3rd or 4th then...
If he fails big certain questions have to asked in the management also they identified Pellegrini as an upgrade on Mancini also a holistic manager, so if Pellegrini fails their big plan also takes a huge hit.
So far we added a quality box to box MF and a fast winger so we improved the squad, so the pressure to deliver is even bigger.
A 3rd/4th place finish in the league and a CL 1/16 final is hardly something we should be delighted with.
Thats what Arsenal is doing in the last few years.
Pellegrini has to be an upgrade in trophies, also in playing better, faster football and also in giving more chance to youth players. Its hard but thats why he is here...