There will be harder nights to explain at Celtic Park, but few more baffling ones. A club that had spent the best part of a fortnight congratulating itself on a composed, clinical 3-0 home performance against LASK of Austria travelled to Linz on Tuesday evening with four goals in the bank and the Champions League group stage essentially within touching distance. By 10 o'clock they were heading for the Europa League, beaten 5-1 on the night and eliminated on aggregate in one of the most stunning collapses in Scottish football's European history.

It had started so promisingly. Callum McGregor drilled a first-time effort in off a defender on 21 minutes — a deflected strike, yes, but coolly struck — and Celtic were 4-0 up across the two legs. The aggregate cushion could barely have been more comfortable. Martin O'Neill's side had other openings in the first half, with Mika Baur stabbing one chance just wide and Kasper Høgh forcing a couple of saves from the LASK goalkeeper.

But Linz had not given up. The Raiffeisen Arena crowd, backed throughout by a vocal home end, sensed something when Benjamin Nygren surrendered possession carelessly midway through the first half. Moses Usor seized on it, and Celtic's lead began to look a little less serene. Three minutes after the restart it shrank further still: Usor muscled past Auston Trusty and laid the ball inside for Robert Ljubicic, who drove home from 20 yards without breaking stride. The tie was beginning to shift.

The turning point that will be replayed in Celtic supporters' nightmares arrived on 58 minutes. Samuel Adeniran, barely given a thought to at that stage, collected the ball on the edge of the box and curled a wonderful first-time strike into the top corner from fully 25 yards. The tie was level on the night and Celtic, suddenly, were trying to manage a situation rather than control one.

O'Neill threw on Liam Scales and Dane Murray alongside Trusty and Cameron Carter-Vickers — four centre-backs, and all the defensive intent that implies — but LASK were not finished. Adeniran fired the underside of the bar, Viljami Sinisalo made a fine stop from Sascha Horvath and the Finnish goalkeeper then saved brilliantly from a Tornich header, only for Florian Flecker to nod in from the rebound deep in stoppage time. Extra time. Celtic, who had arrived in Austria with the job almost done, had to play another 30 minutes.

The additional period could have gone differently. Sinisalo denied Adeniran a penalty shootout from the spot, pulling off a save that briefly revived Celtic. But the striker found his hat-trick goal anyway, collecting the ball 25 yards out after dispossessing Luke McCowan and firing low past a goalkeeper who had run out of miracles. A £20m-plus financial blow confirmed, and the Europa League it is.

The post-mortem will run for some time in Glasgow. Questions were already being asked about the decision to sell Arne Engels in the window before the first leg, with Celtic losing their grip on midfield during the second half in Linz. The club also failed to address their striker situation adequately — a problem that contributed to their exit at this same stage last season, when they drew a blank against Kairat Almaty before going out on penalties.

For all the quality that delivered that first-leg cushion, Tuesday's performance raised the same old doubts about Celtic's ability to see a European job through in difficult conditions. LASK deserve enormous credit; they were relentless, their forwards were outstanding and Adeniran was extraordinary. But Celtic's defensive fragility and their disappearance as an attacking force after the break cost them dearly. The Champions League, and all the money that comes with it, will be enjoyed by someone else this season.

Frequently asked

Why did Celtic get eliminated from the Champions League?
Celtic were knocked out after losing 5-1 to Austrian side LASK in the second leg of their qualifying play-off, surrendering a four-goal aggregate lead they had built from a 3-0 win at Celtic Park. Samuel Adeniran scored twice in the second leg as LASK overturned the deficit in extra time.
Which competition will Celtic play in instead?
Celtic drop into the UEFA Europa League following their elimination from Champions League qualifying.
Who scored for Celtic against LASK in the second leg?
Callum McGregor scored Celtic's only goal of the second leg, a deflected first-time strike on 21 minutes that briefly gave them a 4-0 aggregate lead.