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Celtic lost 5-4 on aggregate to LASK after a 5-1 defeat in the second leg of the play-off and having at one stage held a 4-0 lead over its opponent.

O'Neill's side cruised to a 3-0 win in the first leg before Callum McGregor added another goal to its advantage 21 minutes in on Wednesday.

However, Moses Usor, Robert Ljubicic, Samuel Adeniran and Florian Flecker all scored across the 90 minutes to force extra time, when Adeniran got their fifth to secure a stunning comeback.

It is the second season in a row that Celtic has failed to reach the Champions League proper, and O'Neill acknowledged just how difficult another early exit was to stomach.

"We know we are a million miles away from the finished article; that's absolutely true. We thought that anyway, regardless," O'Neill said. "But we felt that if we qualified for the Champions League, there would be players that would be wanting to come on that very basis. That's not going to be the case.

"But we'll see. When I see that we're short – and maybe talking about a million miles away – we played brilliantly the other night, brilliantly in the game; they [LASK] were a fine side too, scored some brilliant goals.

"And we came up well and truly short – particularly in the second half."

"I'm responsible for this one.

"Without a doubt. Naturally, it took me a long time to get over the UEFA Cup final in 2003, but this was majorly disappointing.

"In the context of things, one was a final which it would have been terrific to win; this was a qualifying game which we should have been able to get through."