Jake Howells’ penalty 10 minutes from time saw Orient fall to a 3-2 Development League defeat against Luton Town.
Sandro Semedo had put the hosts in front with a fine strike after just five minutes, but Orient trailed at the break after Alex Wall and Ross Lafayette netted.
Gianvito Plasmati levelled matters in the second half with a great free-kick, but the hosts were undone when Howells rifled home a spot-kick to seal victory for the visitors.
First-team players Marvin Bartley, Jay Simpson, Plasmati and Shaun Batt all started in a strong Orient side.
But it was under-18s player Semedo who made a big impression as he fired a superb 25-yard shot into the top corner past Hatters goalkeeper Liam Gooch after just five minutes.
It was an action-packed opening and Wall levelled for the visitors on 13 minutes when he drifted in off the right wing and fired an effort across goal into the top corner.
Chances continued to come early on with Plasmati heading off target and Shaun Batt seeing a shot deflected behind for a corner.
The lively Semedo saw another effort go wide in a promising display, before Grainger did well up the other end to keep out a Wall header.
Luton enjoyed a strong spell of pressure and Aron Pollock produced a fine last ditch block to deny the Hatters a second on 28 minutes, but Sam Ling’s game came to a premature end when he was brought down very late by Shaun Whalley. The right-back was unable to continue and was replaced by Nick Brown.
Humphrey and Plasmati couldn’t get on the end of a teasing Lee cross, but as the half drew to a close Luton took the lead when the ball bounced around the box and Lafayette fired home from inside the box.
Simpson came off at half time for Scott Kashket in Orient’s only change for the second half, but the O’s were dealt another blow when Semedo hobbled off through injury early on.
The second period was certainly quieter, but the hosts levelled on 68 minutes when Plasmati’s pin-point 25-yard free-kick found the bottom corner with Gooch motionless in the Hatters goal.
But with 10 minutes to go, the visitors won a penalty for a foul by Marvin Bartley, and Jake Howells’ thumping effort from the spot found the net.
Plasmati went close with another free-kick late on, but Orient slipped to defeat.
Orient: Grainger, Ling (Brown 34), Humphrey, Bartley, Nikolaou, Pollock, Semedo (Moncur 53), Lee, Simpson (Kashket 46), Plasmati, Batt.
Subs: Adeboyejo, Agyemang.
Luton: Gooch, Lawless, Connolly, Parry, Wilkinson (Onyemah 60), Howells, Rooney, Benson, Wall (Lafayette 28), Robinson, Whalley.
Subs: Galliford, Allman.