The Spain international's 89th minute winner was a warning to any incoming striker to Stamford Bridge that he considers himself the main man
The Spain international scored a last minute winner against London rivals West Ham as Conte took charge of Chelsea for the first time in the Premier League
His strike saved Chelsea from a disappointing draw after James Collins struck an equaliser against the run of play to cancel out Eden Hazard's earlier strike.
Conte finds himself still chasing two players despite already kicking off the Premier League campaign, with Lukaku his main target to improve their forward line.
But while it was hardly a vintage performance for the current first-choice centre-forward, he proved he still has the ability to pop up with goals when the Blues most need them.
Costa's strike went against the run of play for him as he frustrated by missing chances all evening, and he should have been sent off for a late foul on West Ham's goalkeeper Adrian.
He will still have to do more to improve his all-round game, having completed a meagre five passes in the first half. Costa's volatile temperament must also come under control, as he received a booking for dissent under new Premier League rules that will oblige players to keep their thoughts about referees strictly personal.
Nevertheless, the goal is the confidence boost the 27-year-old needed, especially after a summer of uncertainty with Atletico Madrid looking to sign him - leaving him distracted during Chelsea's pre-season tour.
Costa was also dropped by Spain for Euro 2016 and was one of many players to suffer poor form under Jose Mourinho, but he has sent a message to Conte that he is still capable of firing the goals required this season.
And if Lukaku does come back to west London, he will certainly have a battle on his hands to take Costa's role at Chelsea.