Debutant Kieran Lee scored a dramatic winner to edge United into the fourth round of the Carling Cup, but Sir Alex Ferguson's holders were given the sternest of examinations by an excellent Crewe display.
The 18-year-old utility player fired home from the edge of the area after a great through-ball from Alan Smith just two minutes from the end of extra time.
United had initially led through Ole Gunnar Solskjaer's sixth goal of the season, only to find themselves pegged back by Luke Varney's second-half leveller for the hosts.
Five members of Brian McClair's table-topping Reserves featured throughout the 120 minutes. David Gray (18) and Phil Marsh (19) both started, while Michael Barnes (18), Ryan Shawcross (19) and Lee all appeared as substitutes.
Gray and Marsh were the youngest starters in a relatively strong United side. The defence bore a particularly fearsome look, with Wes Brown, Mikael Silvestre and Gabriel Heinze all starting ahead of Tomasz Kuszczak.
Namesakes Ritchie and David Jones provided the graft and guile in the centre of midfield, flanked by Kieran Richardson on the left wing and Ole Gunnar Solskjaer on the right.
Marsh and Alan Smith, making his first start since suffering a broken leg and dislocated ankle at Anfield, reprised their recent Reserves partnership upfront.
The relative unfamiliarity of the starting line-up seemed an initial hindrance, with Crewe comfortably on top in the opening exchanges.
Skipper Neil Cox almost flicked the League One side into a fourth-minute lead as his header was brilliantly clawed over by Kuszczak.
The Pole was called into action again on 12 minutes as he smothered Isaac Osbourne's attempt, but it took the intervention of a linesman's flag to prevent Crewe moving ahead four minutes later.
Osbourne's long-range shot was only parried by Kuszczak and Luke Varney was flagged offside as he converted the revbound, much to the chagrin of the home faithful.
It took United until the 24th minute to forge a decent opening as David Jones' low shot was comfortably collected by former United Reserve goalkeeper Ben Williams.
A minute later, however, the ex-Red had no chance as United moved into the lead. Ritchie Jones harried Rix into losing possession before crossing for Solskjaer.
The Norwegian veteran, stand-in skipper for the night, steered a left-foot volley expertly inside the near post to notch his sixth strike of the season and continue his own fine run of goalscoring form.
United's travelling fans were still jubilating the opener when Williams brilliantly denied them a second to celebrate, his reactions to tip Richardson's volley onto the bar were outstanding.
He was called into action again a minute later, smothering at the feet of Solskjaer as United threatened to overrun their hosts.
It was undoubtedly United's finest patch of the game, and one they failed to replicate again in normal time as Crewe gradually edged their way back into the tie.
The dangerous Varney shot narrowly over the top corner after good approach play just after the half-hour, and Dario Gradi's side were still well in the game at the interval.
Michael Barnes replaced Marsh at half-time, and United continued to toil without reward as they aimed to put the game to bed.
Alex kept growing in stature, and it was no surprise when Varney levelled 17 minutes from time. After more good approach work, the striker fired a 20-yard drive through the legs of Brown and inside Kusczak's post.
The goal seemed to wake United from their slumber, and they began to create chances again. Solskjaer and Richardson were denied before the latter ran through on goal, only to fire against the post.
It was the last clear chance of normal time, although United continued to forge half-chances and fleeting openings throughout extra time.
Lee, Richardson and Solskjaer were all denied, while Smith saw his long-range shot well parried by Williams as Crewe continued to defend stubbornly.
United finally made the breakthrough two minutes from time as Gabriel Heinze's punt forward landed at the feet of Smith, whose excellent crossfield ball released Lee.
The versatile youngster bore down on goal and fired a low drive inside Williams' far post to alleviate the threat of penalties and send an exhausted, thoroughly tested United through to the fourth round.
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