He was due to stay at Vicarage Road until January, but the England Under 21 international will see out the campaign with Malky Mackay's side.
Mackay told the club's official website: "I'm delighted to have secured Tom's services until the end of the season.
"The business was once again conducted in a correct and professional manner with Tom, his representative and Manchester United.
"Sometimes it can take time for a player to settle in, but, Tom has hit the ground running.
"That is down to Tom's hard work ethic, his desire and professionalism. It also a reflection upon the club he has come from, while the coaches here have had a big part to play as well since his arrival."