The 19-year-old centre-back made just one substitute appearance for the Robins and has been behind fellow loanee Michael Hector who has started every match since joining from Reading.
"I want to thank Manchester United and Luke," manager Mark Yates told the club's official website.
"Both have been superb and we totally understand why the reasoning for the recall. Luke has had to be very patient but we were happy to have him round here and I hope the experience of being around a first team has been beneficial for him."
McCullough will return to his parent club straight away.