THE words which came from Stuart Armstrong's mouth were superfluous.

Trudging through the labyrinth of corridors deep within the bowels of the Stadio Giuseppe Meazza, the expression on the Celtic midfielder's face told the story a thousand words would struggle to articulate.

It was a night of 'what could have been' for the Scottish champions. Bossing large parts of the game in the opening half hour, Ronny Deila's team looked to visibly grow in confidence in the face of the meagre threat of their Serie A hosts early on, with every passing minute breathing fresh hope into the Celtic players and support.

However, it was an opportunity to dictate their own fate that was strangled in an instant on 36 minutes.

Virgil van Dijk's dismissal by Slovakian referee Ivan Kruzliak for a second booking completely changed the complexion of the tie as Celtic's attacking nous soon became stifled.

Gary Mackay-Steven, the visitors' brightest spark, was sacrificed by Deila and Efe Ambrose was thrust into the heart of a defence that was about to be poked, prodded and probed by a talented Inter front line.

From then on Celtic were restricted greatly in their forays forward while they had Craig Gordon to thank for prolonging a goal at the other end for as long as he did.

While television pictures leave the referee's call to send off Van Dijk open to debate, both the words and the expression on the face of Armstrong claimed otherwise.

"I think we all saw it was a bit harsh," said the man who scored in the first leg at Parkhead.

"I think the two yellow cards weren't merited but they were given, and we had to get on with it. But it was an uphill climb from there.

"I thought we were in the game and had created a few chances. We probably should have tucked one away.

"But of course the sending off changed the game and from then on we were on the back foot and that's how it was for the rest of the game."

While Armstrong's disappointment is undeniable, the frustration at Van Dijk's sending off made up only part of the 22-year-old's angst.

After just 14 minutes the man recently signed from Dundee United found himself in the spotlight as he did last week when he burst into the Inter Milan box.

Getting on the wrong side of Davide Santon, Armstrong looked ready to shoot on goal only to have the full-back lean into his back and send him sprawling on the deck.

Mr Kruzliak deemed it wasn't a spot kick and waved play on.

"Yeah of course I felt contact," claimed Armstrong.

"If there wasn't I'd have taken a shot as there was no need to go down.

"I felt an arm on my shoulder and it put me off balance. It's just the way it's gone and that's unfortunate for us."

Regardless of the refereeing performance, Celtic were still unable to get their noses in front and defend the goal that could have taken them through to the last 16.

It has been a tumultuous journey for the Scottish champions having been knocked out of the Champions League qualifiers twice before making their way in the Europa League.

Armstrong may have only been involved in the final leg of that journey, but it has not lessened his sense of sadness that for this year it is now all over.

"Disappointment is the word. Unfortunately we got nothing out of it," said Armstrong, who was substituted in the second half for Kris Commons.

"There is a lot of disappointed boys in the dressing room and we're feeling hard done by. But it's all done now and we can't do anything."