Guus Hiddink says Diego Costa "likes to fight in a good way" and warned any defenders facing the Chelsea striker that trouble awaits.
Costa scored the decisive goal in Chelsea's 1-0 win at Arsenal on Sunday, not long after being tripped by Per Mertesacker, who was sent off after 18 minutes.
The striker now has six goals in six games under Hiddink, who hopes he will shake off a knee problem in time for Sunday's FA Cup fourth round clash against MK Dons.
Arsenal boss Arsene Wenger again bemoaned the influence of Costa, who antagonised the Gunners last September when Gabriel saw red, but Hiddink was grateful Costa focused.
"He is important, because he's focusing on his first job," said Hiddink. "If you have to play him as a central defender, then you are a bit in trouble.
"He likes to fight - in a good way."
Costa was one of the players who drew the Chelsea fans' anger after Jose Mourinho's departure last month.
Hiddink reckons Chelsea's players have won back the fans. He added: "When I came in I said 'you have to win back the support and confidence of the fans and look to the future'.
"They are pleasing the fans again and the fans are closing them in their arms."