Glasgow head coach Gregor Townsend was frustrated his team failed to hit their best form as they lost 16-11 to Connacht in their Pro 12 semi-final.
An improved second-half performance was not enough as the champions' bid for back-to-back titles ended in Galway.
"We are disappointed we didn't play our best game when it was required," Townsend told BBC Scotland.
"When it comes to a semi-final or final you have to play very well to win and we didn't do that today."
Fly-half Finn Russell was taken to hospital after suffering a "facial and head injury" in the opening minute, receiving oxygen during lengthy on-field treatment.
The 23-year-old stayed there on Saturday night and will remain in hospital on Sunday night for observation.
"It's a nasty looking injury," Townsend told BBC Scotland. "Our thoughts are with Finn and we hope he makes a quick recovery."
Connacht maintained the 100% record of home teams in Pro12 semi-finals as a converted try from wing Niyi Adeolokun and three penalties from AJ MacGinty sealed their passage to an all-Irish final against Leinster next Saturday at Murrayfield.
"In attack we just didn't have the control or the patience to hold onto the ball, to go through that extra phase in the first half," Townsend noted.
"We rushed things a little and weren't as fluent as we can be but in the second half we showed what we can do and came right back into the game but it's a pity we couldn't sustain that."
Veteran winger Sean Lamont said it was "a season's work gone down the drain."
"Credit to the boys, they dug in to the end, but I just think we were on the wrong side of the penalty count and weren't quite clinical enough," said the 35-year-old.
"For the boys that are leaving us it would have been nice to finish on a high but unfortunately we were just a little bit short.
"To lose a couple of guys [Russell and Fagerson] so early it does shift things but I think the guys that came on did a great job. These things happen, it's rugby unfortunately, it's the nature of the sport."
Analysis - former Scotland captain Andy Nicol
"Glasgow will be disappointed with how they played this game for the full 80 minutes. We only really saw what they can do for 10 minutes at the start of the second half.
"Leone Nakarawa was probably their best player and that is what they are losing. He has been outstanding, and a joy to watch.
"If Glasgow had defended their title after the season they have had, losing so many players to the World Cup and the Six Nations, it would have been an incredible achievement.
"Ultimately it was the game two weeks ago which cost Glasgow a home semi-final. You saw the influence the crowd had again, and in a tight game, home advantage makes a big difference.
"Glasgow did remarkably well to get into this position but they just couldn't get over the line.
"Connacht have only lost once at The Sportsground all season and have played some great rugby. You saw the character and resilience and team spirit they showed at the end.
"Leinster will be favourites in the final because of their tradition and history but I would love it if Connacht won it. Their attacking play has been fantastic and it would be great for them to win it for the first time."