Two months after impressing Dennis Wise and his coaching staff during an earlier spell of training with Leeds, the Whites will take another look at the 19-year-old before deciding whether to hand him a permanent deal.
He has already rejected the offer of a three-year contract at Swansea City in an effort to secure a transfer to Leeds.
Striker Aaron Burns, also 19 and a fellow product of Old Trafford's academy, joined Mullan on trial at Thorp Arch in May, and Leeds are still to confirm whether they intend to pursue their interest.