Evans stunned world number 79 Evgeny Donskoy in the deciding rubber to help Great Britain progress past Russia in the Davis Cup last weekend, but Henman believes that the 22-year-old needs to dedicate himself to the sport if he wants to continue his development.
The former Wimbledon semi-finalist told BBC Sport: "There's far better players than him in the world. But he has to maximise his potential.
"What is his potential? I don't know, he doesn't know.
"He has to find that out and the only way he can do that is if he commits, he works and gives 100% to his job."
Evans is currently in 324th position in the world rankings.