The West Sussex side more than held their own against the Premier League leaders and hit the bar in injury time through substitute Richard Brodie. Relief was palpable on the final whistle as United avoided following holders Chelsea out of the draw for the next round.
United manager Sir Alex Ferguson made nine changes from the team that beat Manchester City 2-1, with John O'Shea and Anderson the only survivors. Goalkeeper Anders Lindegaard was handed his Old Trafford debut, while wingers Gabriel Obertan and Bebe were tasked with creating the supply line for lone striker Javier Hernandez.
For the non-league visitors, boss Steve Evans made five changes to the side that started the mid-week 0-0 draw against Wrexham. Goalkeeper Michael Kuipers, captain and centre-back Pablo Mills, midfielder Ben Smith, playmaker Sergio Torres and striker Craig McAllister were all drafted in as the Scot looked to select his strongest side.
Crawley showed no sign of stage fright at the Theatre of Dreams and attacked from the kick-off. A deep cross from Craig McAllister saw Fabio stretch every sinew to nod the ball to safety in the penalty area.
An enormous 97 places separated the teams in the football pyramid but Crawley continued to match their esteemed opponents in the opening stage. After both Obertan and Bebe had threatened out-wide, Smith robbed Michael Carrick on 11 minutes to relieve the pressure on his team.
In a sign of his side's confidence, Smith then swivelled on a throw-in moments later and made Lindegaard sweat on a 35-yard effort that looped narrowly over the cross-bar.
Crawley continued to set the pace as United’s reserves struggled to make their mark. In the 22nd minute, another cross from the left-flank saw McAllister draw a comfortable save from Lindegaard.
The Premier League leaders looked in shock, but their stupor wasn’t to last as Darron Gibson rose to the challenge of preventing a giant-killing.
Close call | Captain Brown spares United's blushes with the only goal at Old Trafford
On 26 minutes, his killer through ball found Hernandez in the area but the threat from the Mexican hot-shot was stubbed out by a fantastic tackle from Kyle McFadzean.
From the resulting corner, the Republic of Ireland international hit a deep cross from the left-flank that was flicked into the corner of goalkeeper Michel Kuiper’s net by stand-in captain Wes Brown's head for his first goal since October 2008.
United now seemed determined to take the wind out of the visitors' sails. In the 33rd minute, Fabio spurned a glorious chance to double the lead when played in by the increasingly influential Gibson, but could only fire wide with Hernandez well placed.
Five minutes later, Obertan tricked his way inside from the left wing and unleashed a thunderbolt that was palmed to safety by Kuipers.
Crawley then slowly started to get back into the match in the final minutes of the half before referee Lee Probert blew his whistle.
If Evans had hoped for some respite in the interval, he was to be denied as last weekend's hero Wayne Rooney was brought on for Anderson.
This change in emphasis almost brought an instant result as Obertan was played in down the left wing in the 47th minute. His low centre seemed destined to be tapped in by Hernandez before Crawley skipper Mills stole the ball from his toe.
Crawley's resilience from the first period was now paid back as they again began to set the agenda. Smart work from Smith and Torres saw small gaps appear in their exalted opponents' rearguard.
Unfortunately for Crawley, the clearest chance of the afternoon fell the way of full-back David Hunt in the 71st minute. The 28-year-old wasted excellent wing play from substitute Brodie as he fired a half-volley agonisingly wide of Lindegaard's right-stick.
The vocal travelling support was at full volume again three minutes later when an acrobatic bicycle kick from Matt Tubbs - reminiscent of Rooney's from last weekend - was adjudged foul play by Probert. The athletic attempt had come after replacement Jamie Cook had twisted O'Shea inside and out.
Crawley continued to show no respect for their opponents and had United on the ropes in the final minutes. Hearts were in the mouths of the home support in injury time as Brodie headed a corner onto Lindegaard's crossbar with the goalkeeper beaten.
Evans' side couldn't quite force a way past the Dane despite their valiant efforts in the closing stages and United's soporific second string was spared a giant-killing.
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