Manuel Pellegrini admitted Marko Arnautovic was "angry" at being substituted during West Ham's 2-0 FA Cup win over Birmingham.

Arnautovic opened the scoring after just two minutes but looked stunned to be taken off by Pellegrini in the 20th minute.

Hammers boss Pellegrini said the Austria forward had indicated that he had hurt his back and was replaced as a precaution.

Yet in bizarre scenes, Arnautovic continually shrugged 'why' as he trudged off, had a lively discussion with Pellegrini on the touchline and stopped in front of the bench to question the medical team before disappearing down the tunnel.

However, Pellegrini said: "It's not very important because we won the game and he scored a goal.

"He felt some pain in his back at the beginning, I thought that it was better to change him because he is just coming off another injury and he played 90 minutes less than 72 hours ago.

"After that he could continue, but I think it was not good to take the risk. He was angry for just one minute, that is it. He wanted to continue.

"Maybe (he signalled) for the change but after that he felt well. He doesn't have a problem, just some pain in his back and nothing more."

Arnautovic had given West Ham a dream start by reacting quickest to nod home after Angelo Ogbonna's header was parried by Blues goalkeeper Lee Camp.

But once he had departed, Birmingham immediately began to show more ambition and finished the first half on top.

Andy Carroll headed a Lukas Jutkiewicz header off the goal-line, Michail Antonio cleared from underneath the crossbar with Gary Gardner waiting to pounce before Maikel Kieftenbeld fired narrowly wide.

It took until stoppage time for Carroll, the man who had replaced Arnautovic, to kill off the Championship side when he headed in Antonio's cross.

It was Carroll's first goal of the season after yet another campaign interrupted by injury.

The striker, who will be out of contract in the summer, has been linked with a January move away.

But Pellegrini, who also saw Samir Nasri put in a lively debut, said: "I never had any doubt that Andy Carroll wants to stay here, he doesn't want to go anywhere.

"There were a lot of positive things today. One of those is the minutes that Nasri played after a long time without playing games. Also we won and Andy Carroll scored."

For Birmingham, it is back to the grind of their play-off push and manager Garry Monk said: "We know the demands on this squad, we only had 18 fit players, it has been a huge effort.

"We didn't get the result we wanted but we wanted to compete and we did.

"We threw everything we could at them but just lacked a cutting edge. I can't criticise the players at all."