Andreas Pereira's deflected winner earned a fifth-round tie at Huddersfield Town but only after his namesake, Joel Castro, pulled off some vital saves in goal. United allowed the Foxes a couple of late attacks in the dying seconds with McGuinness singling out centre-back Ryan McConnell for special praise.
However, the Under-18s boss was keen to stress the importance of gaining experience and avoiding the need for any last-ditch defending so deep into crucial matches. The absence of leading strikers James Wilson and Ashley Fletcher was also an issue in terms of rewarding sustained pressure with dangerous efforts on goal.
"It was a great win to go through but I think we made harder work of it than we needed to," explained McGuinness to MUTV. "But we dug in and had some fantastic performances at the back. Ryan McConnell was terrific, heading everything away, as were all the back four and the keeper made a couple of terrific saves.
"But I don't think that's the full story of the game. I thought we had control of most of it but missed our forwards to have that finishing touch. We made two or three bad decisions around the box where we should have passed instead of shooting and that cost us a lot as we went in to extra time rather than being one or two goals up.