Celtic suffered a damaging and likely decisive Europa League knockout round play-off first-leg defeat to Stuttgart as Martin O'Neill's 1,000th match as a manager ended in disappointment.

Bilal El Khannouss' first-half double sandwiched Benjamin Nygren's cool finish and put Stuttgart in control as they punished defensive fragility to leave the home side with a mountain to climb to progress.

Goalkeeper Kasper Schmeichel had a night to forget, looking at fault for the first and third goals as he tamely failed to stop Jamie Leweling's second-half strike that earned Stuttgart a commanding cushion.

A fine late fourth finish from Tiago Tomas ended Celtic's 10-game unbeaten run since Wilfried Nancy's exit and leaves them with little hope of progression.

Under O'Neill, Celtic had produced some memorable displays and results in this competition.

They struggled to hit those heights despite playing well at times and now travel for the return leg against a side who sit fourth in the Bundesliga needing to record a first-ever win in Germany.

O'Neill stated in the build up that this would be a very tough test. It proved that but, in truth, Celtic's biggest obstacle seemed themselves.

They looked comfortable until Schmeichel was short with a clearance. The ball found El Khannouss whose reversed strike seemed tame, but the goalkeeper looked very laboured as he failed to get down to save.

Stuttgart captain Atakan Karazor was equally accommodating shortly after as he gifted Celtic a lifeline when he passed straight to Nygren who did superbly to round the goalkeeper and tap home.

That was his 18th goal of the season - a spectacular return from the Swedish midfielder.

Just as the home crowd sensed that reprieve may inspire something, they were undone once more. A cross deflected kindly to El Khannouss all alone in the penalty area and he planted a header home.

It was all far too easy.

Not for the first time of late, O'Neill was left with a half-time rousing mission to get Celtic back on track.

There was a response, but again Schmeichel looked at fault when Leweling struck from outside the box.

A fourth was ruled out when Ermedin Demirovic lifted home but the video assistant referee (VAR) deemed him offside to Celtic's relief.

Nygren was close to reducing the margin as the home side performed well enough and ploughed on but they were caught out in injury time when Tomas ghosted in to loft home.

The damage was done in those defensive moments with no miracle under O'Neill on this occasion.

Celtic have revelled in doing it the hard way of late. Defensive frailty is the reason and it cost them once again. This time they couldn't respond, albeit they tried.

They need to make astonishing history by earning a first win in Germany by a big, big margin. Not many will back this scoreline being overturned.

Memories of Munich in last season's Champions League play-off may offer very slim hope but Celtic seem a stretch away from that level of performance right now when they agonisingly lost out to Bayern's late, late leveller.

The chance to progress seems gone.

The display, overall, was not bad. Stuttgart didn't actually create that much for the most part but a late push looks to have got them over the line.

A lot of this outcome was down to Schmeichel. The are increasing question marks around Celtic's goalkeeper.

His distribution for the opening goal was poor but his dive for the strike looked laboured to say the least. The Denmark legend has looked less than comfortable this season.

Collectively, the second concession was also weak with El Khannous inexplicably left alone as Celtic sat far too deep.

The third was simply a shot that Schmeichel should have saved and the late concession making this look a bridge too far.

Progression now is a very tall order against a side who are very strong on home soil with the weight of history very much not in Celtic's favour.

Celtic interim manager Martin O'Neill tells TNT Sports: "It's a tough evening for us. On paper, 4-1 looks pretty poor. We were masters of our own downfall in many respects, conceded some poor goals.

"Kasper [Schmeichel] has made some great saves while I've been here. This is a collective, there's no doubt about that.

"I will think about all sorts of things, but Kasper has done really well since I've arrived at the football club, in both spells."

"At some point, Celtic will become a top quality European team. That's in the future. At this moment our battles are elsewhere. Our focus now is totally on Sunday.

"It's very difficult for Scottish teams. There's no money. You're talking about Premier League sides and Bundesliga sides buying players for £40m and them not playing."

Celtic return to the defence of their Premiership title when they host Hibernian on Sunday, 22 February (15:00 GMT), before the return leg against Stuttgart on Thursday, 26 February (17:45).