Games against teams such as Cardiff City could prove costly if Pellegrini does not bolster defence, writes Richard Jolly.
Bogey side issues at start of the season are back to haunt Manchester City
For Cardiff City fans of a certain vintage, Sunday's 3-2 victory over Manchester City was like being transported back half a century, to their only previous top-flight league wins.
For the other City, rather less time travel was required: this was a defeat with disturbing echoes of last season.
Progress is not always a smooth, upward curve, and as emphatic as City's opening win was, there was no sense the austere Chilean assumed that his work was done and the transformation of a club was complete.
Instead, he received an unwanted reminder of underlying issues City will have to address if they are to reclaim the Premier League title.
Even as Etihad Stadium remained a fortress last year, former coach Roberto Mancini's team had an unfortunate habit of underperforming in certain away games.
Everton, Swansea and Sunderland were all bogey teams on their own turf. Perhaps now Cardiff can be added to that list.
Set-piece troubles were a recurring theme last autumn. After a loss to Ajax, an exasperated Mancini exclaimed that he should not have to tell Joleon Lescott to jump when defending a corner.
Following the loss at Cardiff, Pellegrini, shrugging off questions about his unfamiliarity with the British game, pointed out that defending dead-ball situations is similar the world over and that a team cannot afford to concede twice to them, as his did.
Another unfortunate action replay came as Joe Hart erred for Cardiff's second goal.
The Englishman's mistakes have become increasingly frequent in the past year; before his competitive debut as manager, Pellegrini found himself having to defend Hart after a blunder against Scotland on international duty.
His is a situation similar to Pepe Reina's at Liverpool, where a goalkeeper, seemingly at the peak of his powers, goes from saving his side points to costing them. Brendan Rodgers came to the conclusion it could not be dismissed as a blip and replaced Reina.
City are not at the same stage, yet, with Hart, whose problems appear more mental than technical. Roy Keane, with typically lacerating criticism, branded Hart, 26, as "cocky" 10 months ago and the chances are that Mancini would concur, after his relationship with Hart broke down completely.
The goalkeeper presents a test of the more diplomatic Pellegrini's management skills.
Mancini complained long and loud last season about City's reluctance to strengthen swiftly and sign his preferred transfer targets, and he no doubt was annoyed when his successor was allowed to spend the best part of £90 million (Dh514.9m) on four A-list recruits with six weeks of the transfer window remaining. But their spending spree had one anomaly.
City were strangely slow to act when Pellegrini requested an extra central defender.
Interest in Real Madrid's Pepe was registered almost two months ago and they have dallied over a deal for Atletico Madrid's Martin Demichelis, who was available on a free transfer earlier in the summer.
Given football's uncanny ability to punish the poorly prepared, it felt inevitable that Matija Nastasic and Vincent Kompany incurred injuries.
The Belgian's deputy, Javi Garcia, an indictment of last summer's desperate late dash for signings, is a midfielder, anyway, and endured a hard time against Cardiff's Fraizer Campbell.
That Kolo Toure, Kompany's reliable understudy, was allowed to leave City and is now in excellent form for Liverpool merely compounded the feeling this was an all-too-avoidable accident. While Pellegrini, having been named manager only 10 weeks ago, was more observer than cause of defeat, he can be faulted for his substitutions.
Removing Jesus Navas deprived his side of width; replacing Edin Dzeko saw the most in-form striker depart.
Mancini's changes, too, tended to be controversial. In every other respect, Pellegrini has been the antithesis of the Italian: he has created a happier camp, placating unsettled souls like Dzeko and Samir Nasri, and has been almost deliberately dull in his public utterances, whereas his predecessor could not stop himself from making headlines.
Yet while there is a very different face to Manchester City, the makeover will be only cosmetic until they can rid themselves of the flaws that cost them last season. Because, if you scratch below the surface, the new City may still look too much like the old.
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