Manchester City manager Manuel Pellegrini revealed Sergio Aguero was not fully fit after his striker's stunning five-goal haul against Newcastle.
Aguero destroyed the Magpies by scoring five times in a blistering 20-minute spell as City ran out 6-1 winners at the Etihad Stadium to power back to the top of the Barclays Premier League.
Aguero had gone into the game with question marks over his form after scoring just once in open play in 10 previous appearances this season and a patchy display against Borussia Monchengladbach in midweek.
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But his late penalty winner in Germany on Wednesday appeared to have rejuvenated him and Newcastle found him too hot to handle.
It then came as a surprise when Pellegrini substituted him after 66 minutes - denying him the chance to become the first player in Premier League history to score six in a game - but Pellegrini revealed he had been carrying a knock.
Pellegrini said: "He had a serious kick from Wednesday in Germany. He was having some treatment during half-time.
"To give him more minutes, maybe he would have scored one or two goals more, but we need more to take a precaution about his kick because he could get injured."
When asked more about the nature of his treatment at the interval, Pellegrini said: "Maybe it was very good treatment!
"He had a kick in his hamstring, so it was important just to relax him a little bit because he was not 100 per cent comfortable."
Pellegrini was convinced Aguero, City's top scorer last season with 32 goals in 41 appearances, would soon come good.
Aguero did score against Chelsea in the second game of the season but then had a setback after suffering a knee injury in a challenge from Crystal Palace's Scott Dann last month.
Pellegrini said: "I am very happy for Kun. He deserves it. He is always a player who makes a difference.
"For different reasons in the last few games he missed important chances, but he was calm. That was very clever from him. To see him play at the level he played today, not only must I be happy but all the fans because he is a different player."
Aguero's stunning performance came after City had fallen behind to an early strike from Aleksandar Mitrovic.
Newcastle, who slipped to the bottom of the table following the loss, were actually the better side in the first half and had other chances to extend their lead.
Manager Steve McClaren said: "We came here to win. We were bold in our 4-4-2 in the first half we really shook Man City, and scored the first goal.
"The second goal that was disallowed was onside, so we should have had that and we had other opportunities. It could have been a different story.
"The goal before half-time just gave them a lift and in the second half we just got done by mistakes by ourselves and Aguero's world-class finishing, which he is well capable of doing to any team."