The 20-year-old attacker made three Championship appearances early last season during the managerial tenure of Matt Taylor but has since been out of the first-team picture at AESSEAL New York Stadium.
A loan move to Rochdale, who sit fourth in the division directly below the Football League, had been fixed up this summer only for the Northern Ireland youth international to be sidelined by an ankle issue.
McGuckin, who came through the Millers’ youth ranks, missed all of the pre-season program but is fit again and in need of senior match action to accelerate his development.
“The plan will be for him to go and get some games,” boss Steve Evans said.
“He was all set to sign for Rochdale on loan and then twisted his ankle when he was training with the youth team.”
The player, who can operate centrally or out wide, signed his first professional deal in 2022 when Paul Warne was in the hot-seat.
He had a successful loan at Scarborough Athletic in the 2022/23 campaign when he was a regular scorer for the National League North outfit.
Last term, a spell with League of Ireland Dundalk was affected by injury and he played only a handful of times.
The Millers activated an extension clause in his contract in May, tying him to the club until the end of this season.
