Mason scored his third goal of the season as Derby earned a 2-0 win over Preston.
"He can be really good, he can be top, top level because he's 19 and plays with ability and quality, technically he's brilliant but also his work-rate shouldn't go unnoticed.
"He should never be considered a number 10 because there's so much more to his game but he'll score goals like a number 10.
"At some stage he will go back to Chelsea. I don't want to think about that now, but when he does I think he's going to be a contender."