The midfielder is one who Maresca sees as part of his first choice lineup, even when all players are fit.
But that does not mean the Italian will tolerate a drop in standards from any player.
"Romeo is doing very, very good with us but because I know him very well, he can do much better," the Chelsea head coach said on Tuesday.
"So I see Romeo at the moment, I'm very happy with Romeo, but Romeo can do better because he is a top player.
"He is 20-years-old and he has to improve many, many things. We are very happy with him but he can improve many things."
He added: "I trust him, for sure. Romeo, he didn't play Leicester away, so two games ago he was on the bench. I say always, because he is on the bench it doesn't mean we are not happy and he is not playing well. It depends on the game-plan."
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