As Manchester City strolled to a 3-0 win over Viktoria Plzen on Tuesday night, they seemed to muster the confidence and desire that was clearly lacking during Roberto Mancini's tenure at the club.
However, they also showed the defensive frailties that have become synonymous with Pellegrini's City in the Czech Republic, and that was with the returning skipper Vincent Kompany in the heart of the back four.
It is this that could cost them a place in the knockout round, which is the minimum aim expected from the new man in charge.
Stronger sides than Plzen will hone in on these weaknesses and exploit them ruthlessly. If they afford Bayern Munich some of the chances they gave the Czech side, they could be in for humiliating defeats.
Even CSKA Moscow, who many feel won't cause City too many problems at home or away, will be a lot more clinical than the Czech champions, and it is they who Pellegrini's men are likely to contest the second spot in Group D with, therefore victory at the Etihad and a point in Moscow are vital to their qualification hopes.
They won't achieve that unless they sure up at the back, that's for sure.
Alexsandr Kolarov was found wanting on too many occasions to count, Matija Nastasic looked shaky at best, and they can't continually rely on the exceptional Kompany to carry them through games.
Some might argue that their attacking prowess might be enough. Granted City possess a formidable frontline that should provide goals-a-plenty during the group stages, but if they can't keep them out, their goals at the other end could be in vein.
Dzeko, Aguero et al are likely to find the net more often than not, especially in their home games. But their clear deficiencies that were highlighted by Plzen on Tuesday could see Pellegrini follow Mancini's exit route out of the group stages.
Add to that tough trips to Munich to face the Champions League holders, an always tricky journey to Moscow and increased pressure at home, and Pellegrini might be waving goodbye to the competition he loves a lot earlier than expected.
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