Manchester City booked their place in the last 16 of the Champions League but there were more concerns for Pep Guardiola over his side's lack of ruthlessness in a 1-0 win at Olympiakos on Wednesday.
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Phil Foden's low strike nine minutes before half-time ensured City deservedly kept up their 100 percent record in Europe this season.
But a one-goal margin of victory was scant reward for the visitors' dominance even with a heavily-rotated side.
Guardiola made five changes to the team beaten 2-0 at Tottenham on Saturday to see City off to their worst league start since the first few months of the club's Abu Dhabi-backed ownership.
Scoring goals has been City's big problem in the Premier League, but they had scored three in each of their opening three Champions League wins over Porto, Marseille and Olympiakos in Manchester three weeks ago.
The lack of killer edge was in clear evidence in Piraeus. City had 21 efforts on goal, 10 of which were on target, but again missed Sergio Aguero's presence in the final third.
Aguero started on the bench as his former father-in-law Diego Maradona was remembered with a minute of silence before kick-off.
Maradona, whose daughter Giannina was married to Aguero, died at the age of 60 on Wednesday.
Once the action got underway, City soon settled into their stride.
Jose Sa denied Gabriel Jesus when the Brazilian was clean through on goal and twice made good saves to deny Raheem Sterling from outside the area.
Sterling turned creator for the only goal with lovely backheel into the path of Foden, who drilled home his fourth goal of the season.
Chances continued to come and go for City after the break. Ilkay Gundogan blasted inches wide before Joao Cancelo and Bernardo Silva shot straight at Sa with the goal at their mercy.
But one goal was enough as Olympiakos failed to muster a single shot on target, complicating their chances of reaching the knockout stages.
The Greek champions trail second-placed Porto, who travel to Marseille later on Wednesday, by three points.
City just need a point from their final two matches to secure top spot in Group C.