The Reds needed cover out wide with Adam Hammill ruled out of the next three games through suspension.
“I’m delighted to have Harry join us,” said Heckingbottom. “He is an exceptional talent and is very highly thought of at Middlesbrough. We’re very grateful to them for allowing him to join us.
“I’m sure he can prove valuable for us as we head into tho final run-in, and hopefully we will prove valuable to him in his development as a promising young footballer.”
Defender Kevin Long has returned to Burnley following his loan, Aidan White is out with a serious knee injury, while Hammill starts a three-match ban starting tomorrow at Crewe Alexandra.
Caretaker head coach Heckingbottom said: “It’s always a concern to lose experience and two of our better players this season, but it’s an opportunity for someone else.
“George Williams has stepped up at left-back and is trying his best to keep in the team, putting his body on the line and that’s what we expect from whoever comes in on Saturday.
“Aidan has had his scan and there’s a rupture of one ligament, the least important as such, but there’s a grade two tear to the more important ligament.
“We’re going to treat it conservatively at the minute through rehab and the specialist was happy that he didn’t need surgery.
“Aidy has been great looking after his body and in terms of quality and we’ll have him back as soon as possible, we’re optimistic he’ll play again this season.”
“With Adam, you can see why he’s been given a red card but there is nothing we can do about it. It’s pointless going for a review because they’ll only need a freeze-frame and contact point.
“It’s unfortunate we’re going to have to lose him for three games, Adam’s as disappointed as we are, and he should be because he knows better but we’ll get on with it.”
Heckingbottom is keen to sign a loan winger today, given the loss of Hammill.
He said: “We are looking and, hopefully, we can have a winger in before Saturday because we’re looking light again as a squad.
“Although you might be happy with your 11, we need to make sure that we’re strong enough to cope with anything that might come about on Saturday and before Tuesday as well.”
Three clean sheets in Barnsley’s last four games have been the base for the Reds’ success in recent weeks.
“It’s absolutely huge,” said Heckingbottom.
“Despite being frustrated in how we performed going forward on Tuesday, at no point did I feel threatened.
“That’s a credit not just to the back four and the keeper but the whole team, because we attack as a team and defend as a team and we defended great.”