Celtic were knocked out of the Champions League on Wednesday when a 2-1 home defeat to Ferencvaros of Hungary meant that for the first time in 15 years they failed to even make the third qualifying round.
“It’s anger and frustration,” said Celtic coach Neil Lennon.
The Scottish champions were hampered for the single-leg tie by the absence of leading striker Odsonne Edouard with a thigh injury.
David Siger shot the Hungarian side into the lead in the seventh minute of the second qualifying round tie behind closed doors at Parkhead.
Ryan Christie, deputising for Edouard, levelled in the 53rd minute with a deflected shot.
Despite dominating possession, Celtic were undone when Tokmac Nguen scored a 74th-minute winner on a breakaway.
“The game was actually a lot easier than what I thought it was going to be, but we didn’t take our chances. We had plenty of them,” Lennon told BBC Scotland having seen his side take 23 shots at goal.
“The first goal we can maybe legislate a little bit, the second goal is really poor decision making.
“It’s individual mistakes. It’s cost us again tonight.
He added: “It’s not fine going out at this stage of the competition, because we’re better than that.”
While Ferencvaros, coached by former Tottenham and West Ham striker Serhiy Rebrov, can look forward to a final qualifying round clash, Celtic drop down to the Europa League qualifiers.
Ferencvaros arrived in Scotland with a proud record on the road in Europe.
They didn’t lose in seven away games in the Champions League qualifiers or Europa League last season – five draws and two wins.
Elsewhere, Dinamo Zagreb won a penalty shoot-out 6-5 at Cluj in Romania after the two sides were locked 2-2 after extra-time.
On Tuesday, 1991 European champions Red Star Belgrade edged Albania’s Partizan Tirana 1-0.