The on-loan Millwall striker marked his debut with the only goal of the game in the London derby, glancing home Marcus Bean's header to double his tally for the season.
But Grabban, who hasn't played for the Lions since February 6, insists he can get even better.
He said: "I've still got some fitness and match sharpness to improve on. There's still more to come once I get into my groove.
"It was a good debut, and one of the best starts I could of hoped for."