West Ham were awful in the first half against the Saints, and went into the break 1-0 down after Dusan Tadic had put the south coast club in front.
Bilic introduced Carroll at half time, though, and the target man’s performance helped his side turn the match around.
Michail Antonio scored a fortuitous equaliser for the Hammers, before Carroll nodded in the winner.
And Bilic said of the England international forward: "I think he was best for us this season when he was coming on [as a substitute] so as an impact player. So maybe that is what he has to do also with the big long break that he had.
"His impact against Norwich, against Chelsea, or today it's not only because of the goal. He is making spaces, he is winning the ball, he is working really hard and he can also pass the ball so he was also one of the reasons we turned the game around in the second half.”
West Ham’s win over Southampton was their first in eight Premier League matches.
Bilic’s men have been ravaged by injury, with their attacking options looking limited at the moment.
With Manuel Lanzini, Diafra Sakho and Dimitri Payet all out injured, West Ham have struggled going forward, but a more direct approach paid dividends against Southampton.
The Hammers are now in seventh place in the Premier League table, while the Saints are 12th.