An early goal from Crawley’s Billy Clarke condemned Orient to defeat and completed a week to forget for Russell Slade’s side.
In the space of eight days two successive league defeats have seen the O’s slip to 13th place in the table, 12 points off the play-off places, and out of the Johnstone’s Paint Trophy.
On a bitterly cold afternoon, the O’s boss made three changes to the side that had drawn 2-2 at Roots Hall on Wednesday night.
Skipper Nathan Clarke was restored to the starting 11 for the first time since picking up a foot injury at Portsmouth in November.
He returned to the heart of the Orient defence with Scott Cuthbert moving to right-back, while Romain Vincelot made his first start in the centre of midfield and Charlie MacDonald was given the selection nod after being cup-tied on Wednesday night.
Moses Odubajo and Shaun Batt dropped to the bench, while Martin Rowlands was missing with a groin injury.
The O’s, desperate to get back on track, could not have scripted a worse possible start as it was Crawley who grabbed the lead with an out-of-the-blue effort on just 11 minutes.
There seemed to be no danger when striker Crawley striker Clarke gathered the ball wide on the right, but he sent in a speculative effort from the edge of the area, that curled past Orient keeper Jamie Jones and into the far top corner of the net.
Little was seen of Crawley as an attacking force after that, but they almost stretched their lead on 20 minutes when a cross from former Orient loanee Nicky Adams found Mike Jones eight yards out and his header hit the top of the bar.
But for the majority of the first half O’s enjoyed the majority of possession without creating too many clear-cut openings.
O’s defender Mathieu Baudry had a couple of half-chances with headers, the first from a Leon McSweeney cross that drifted just over on 15 minutes and another that visiting keeper Paul Jones saved easily from a Dean Cox free-kick.
But it was the defender’s header just before the break that almost gifted Crawley a second goal.
Baudry attempted to head the ball back to keeper Jones, but it fell into the path of Clarke, who failed to re-discover his early goal scoring touch, and sliced his effort horribly wide.
Orient’s best chance of the first half fell to MacDonald, who latched onto Cox’s flick, powered into the area and sent in a powerful drive that keeper Paul Jones managed to push away on 28 minutes, while 60 seconds later Leon McSweeney tried a spectacular volley from 40 yards that looped inches over the bar.
In a bid to get back into the game, the O’s boss made a change 11 minutes after the break when Odubajo replaced Cook, with Cox switching to the left.
But as Crawley started to stamp their authority on the game, with Adams looking dangerous, the O’s boss made a second change just after the hour when Shaun Batt came on for MacDonald.
The move seemed to give Orient more impetus and they almost levelled 20 minutes from time when David Mooney wriggled down the left and cut the ball back for Cox, but his shot from eight yards was blocked by Matt Sparrow and flew behind for a corner.
Leading scorer Mooney then headed a Vincelot cross wide of the target on 78 minutes, before he was replaced by Jimmy Smith.
As Orient continued to push forward, defender Cuthbert saw his powerful header from Cox’s corner blocked by keeper Jones, before the lively Odubajo shot wide.
In the fourth minute of stoppage time Orient almost salvaged a point when Cox sent a free-kick from 25 yards inches past the left-hand post but, despite their late barrage, Orient were unable to find a way through the Crawley defence and slipped to their 15th league defeat of the season.
Orient: Jones; Cuthbert, Baudry, Clarke, McSweeney; Cox, James, Vincelot (J Smith 78 mins), Cook (Odubajo 56 mins); MacDonald (Batt 62 mins), Mooney. Subs: Grainger, Griffith, Obafemi, Omozusi.
Crawley Town: P Jones: Dumbuya, Connolly, Walsh, Sadler; M Jones, Sparrow, Bulman, Adams (O’Brien 88 mins); Clarke (Torres 82 mins), Proctor. Subs: Maddison, Essam, Hunt, Elford-Alliyu, Smith.
Goal: Clarke (11 mins)
Bookings: Proctor, Sparrow
Referee: Steve Rushton
Attendance: 3,465 (inc 527 away)