Marko Arnautovic and Andy Carroll scored as West Ham beat Championship club Birmingham 2-0 in the third round of the FA Cup on Saturday.
Arnautovic put the Hammers ahead in the second minute, reacting quickest to nod home after Angelo Ogbonna's header was parried by goalkeeper Lee Camp.
But the Austrian striker was far from happy at being substituted after only 20 minutes.
Pellegrini: Arnautovic was angry
In bizarre scenes, Arnautovic continually shrugged 'why' as he trudged off, had a lively discussion with manager Manuel Pellegrini on the touchline and stopped in front of the bench to question the medical team before disappearing down the tunnel.
Pellegrini said afterwards that he wanted to avoid any "risk" after the 29-year-old had complained of "some pain in his back".
Andy Carroll, the man who replaced Arnautovic, made it 2-0 in the 90th minute when he scored his first goal since April.
Samir Nasri, the former Arsenal and Manchester City midfielder, played 60 minutes on his debut for West Ham, having joined after completing a doping ban.