Kris Commons admits Celtic were well below their best as they suffered Co-operative Insurance Cup heartache.
The Bhoys entered Sunday looking to land a domestic Treble, but ended it chasing a Double.
A 2-1 defeat to Old Firm rivals Rangers at Hampden Park saw them miss out on the first silverware of the season.
An extra-time effort from Nikica Jelavic took the trophy for the Gers and Commons conceded afterwards that Celtic could have few arguments.
"We have let down ourselves, the fans, the coaching staff and especially Neil (Lennon)," he said.
"There was six or seven lads who didn't turn up today. I don't think we did enough to win the game or lose it.
"There were some okay performances, a few of us didn't play to our abilities and we switched off certainly for the second goal.
"In the build-up we felt confident and sharp but today there was no evidence of that."
Commons is now hoping that Celtic can get the job done in the league, where they hold a two-point lead over Rangers.
He added: "If we concentrate on our job and do what we can do best in winning matches then I think we will win the league.
"But I think a lot of people need to take this in and reflect on this game and try to use this as momentum to kick on."
Meanwhile, assistant manager Johan Mjallby admits Celtic paid the price for some sloppy defending on cup final day.
He told Sky Sports: "It's always difficult to take when you lose a cup final, especially in extra-time. But all credit to them. We should have probably done a bit better with the two goals we conceded. That is disappointing."