Mellor has featured in the Railwaymen's last six matches, starting four, as they have stormed into the final League Two play-off place with two games of the season remaining.
He was called upon from the bench last weekend to replace injured skipper David Artell and help see out the vital 1-0 win over Cheltenham, and Davis is impressed with the 21-year-old defender's growing composure
"Kelvin has done exceptionally well, I have to say," said the Alex boss. "We were obviously worried about him over the last couple of seasons that he was a little bit nervous and wasn't going to step up to it, but he has done and he's shown he's more than capable of holding his own and performing very well at this level.
"So that's a plus point for us, we're certainly not afraid to put him in at full-back if we need to - I don't see him as centre-half at the moment, maybe in the future.
But certainly as a full-back, left or right, he can do a good job.
"He's a big lad who's good in both boxes and he's got a lot of composure and a lot of qualities in terms of his passing. He's given us something that we didn't expect."