The Scot’s loan from Rotherham United included a clause which prohibited him from featuring in the competition.
It was thought the Millers’ defeat at Leeds United on Saturday had prompted the U-turn, but U’s boss Michael Appleton revealed it had been arranged before the weekend and kept under wraps to keep Swansea City guessing.
Appleton said: “What you don’t know is the call was made way before the weekend.
“We wanted to keep it under our hats.
“There weren’t too many surprises we could have given to Swansea, but that might have been one of them.
“We didn’t want to let it out of the bag too early.
“He was magnificent, he showed his class that he is better than a League Two player, no doubt about it.”
It turned out to be an inspired move, with Maguire starring in the 3-2 victory.
The 26-year-old was excellent throughout and played a key role in laying on Kemar Roofe’s second goal, which turned out to the the winner.