The game finished 1-1 after extra-time before Boro won 8-7 in the shootout when United forward Anthony Elanga fired his spot kick over the bar.
Ralf Rangnick’s side missed the chance to open the scoring when Anfernee Dijksteel fouled Paul Pogba in the box, but Cristiano Ronaldo missed the subsequent penalty in the 20th minute.
The hosts did go ahead five minutes later, though, when Jadon Sancho’s deflected effort beat Boro keeper Joe Lumley.
United had more opportunities to extend their lead but were pegged back in the 64th minute when Matt Crooks bundled home an equaliser to force extra-time.
Here’s how each Boro player fared at Old Trafford:
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. MANCHESTER, ENGLAND - FEBRUARY 04: Matt Crooks of Middlesbrough is challenged by Paul Pogba of Manchester United during the Emirates FA Cup Fourth Round match between Manchester United and Middlesbrough at Old Trafford on February 04, 2022 in Manchester, England. (Photo by Clive Mason/Getty Images)
Matt Crooks. Photo: Clive Mason

. Joe Lumley - 6
Almost gifted Bruno Fernandes a goal in the second half but also made some important saves when he was quick off his line. 6 Photo: Robbie Jay Barratt - AMA

. Anfernee Dijksteel - 6
Recovered after conceding an early penalty. Gained more confidence when stepping out of defence with the ball. 6 Photo: Getty Images

9. Marcus Tavernier - 6
Found it challenging to get into the game operating just behind the front two.Kept working for the team and made some late defensive contributions. 6 Photo: Getty Images

10. Andraz Sporar - 6
Managed to see more of the ball in the second half when he was able to drop deeper and link up play. 6 Photo: Getty Images

11. Folarin Balogun - 5
Didn’t see much of the ball up front and struggled to trouble the United centre-backs. Was subbed after 61 minutes. 5 Photo: LINDSEY PARNABY

12. Duncan Watmore (sub) - 7
(for Balogun, 61) Gave Boro an injection of energy they needed and assisted the equaliser, even if the ball hit his arm. 7 Photo: Getty Images