City were coming off a dominant 3-0 win at Chelsea on the weekend but failed to take advantage of that momentum when producing a fairly lacklustre showing at St James' Park.
Sergio Aguero opened the scoring with a 14th-minute header before Vernon Anita replied just after the half-hour mark in what Pellegrini suggested was a discouraging display after such an inspiring exhibit on the weekend.
"That was not the same team as on Saturday, without any doubt. Maybe we couldn't recover so soon after three days to have the same pace that we had against Chelsea," he said.
"We knew also before the game that it was going to be a very tough game because Newcastle are playing not to be relegated next season. They had just won their last game, so it was a difficult game as we expected."
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