McCourt has made four appearances for the Gulls since moving to Plainmoor in October.
United boss Alan Knill told the club's official website: "Jak is a good player and we've extended his loan by another month.
"For someone so young, he's pretty mature on the pitch.
"Sometimes you forget that he's 18, and that is a compliment to him because I don't think there are many of his age playing in the league.
"He's quite critical of his own game but if he can get 30 league games by the time he's 19 (next July), that is pretty good on his CV and I only think he's going to get better.
"He gives you honesty and athleticism. When it's near the end of the game and he's still breaking into the opposition box, that has to be good, and that's what youth gives you."