Sky Sports report Mellor is set to undergo a medical and all the paperwork is expected to be done within the next 24 hours.
The attacking 21-year-old went on loan to Salford City at the end of the summer transfer window but only managed three appeaRances.
He scored two goals for the under-23s last week in a 2-1 victory over Brighton.
www.unitedinfocus.com said: "Mellor, 21, needs to go out and play as he has an uncertain future at United with his contract set to expire, and little chance of regular first team action, barring a fantastic goalscoring spree out on loan at Crawley."
The Reds are currently missing top scorer Kwesi Appiah with an injury.
Crawley Town travel to Bradford City tomorrow (Saturday.