Aberdeen sent Stuart McCall's men spinning out of the tournament last Wednesday with a 2-0 win despite playing almost 75 minutes with 10 men.
The Fir Park side bounced back as McManus' header beat managerless Hibs in the Scottish Premiership but the former Celtic skipper said he was still feeling blue following the Dons defeat.
With both Celtic and Rangers out of the competition, it offered the Steelmen a rare shot at silverware but McManus admits they blew their big chance.
He said: "In all my career that's as low as I've felt after a game to be honest. It was a real kick in the teeth. The manager said that as well.
"I've been fortunate enough to win a few trophies here and there during my career.
"But when you look at the club, Motherwell have been successful under Stuart McCall's leadership yet they've not managed to win a cup.
"I think 1991 was the last time that happened. For a smaller club, you believe you've got a better chance when Rangers and Celtic are out of the competition.
"It's the best chance you can ever have of getting to a final and you never know what might happen when you get there.
"That's why it's bitterly disappointing. Aberdeen are the strongest side in the competition along with ourselves and Inverness.
"When they go down to 10 men early you are saying to yourself that you've got a right good chance.
"We dominated the game but when you look at all those aspects and then look at the way we lost it. It's hard to take."
There was consolation at least for McCall's team as their 1-0 victory over Hibs took them to joint second in the Premiership alongside Inverness.
"I'm just pleased for us and the manager," McManus added. "It was so important after the other night.
"Especially with the situation with Pat Fenlon leaving, we knew Hibs were going to come and play on the front foot.
"We knew it was going to be difficult especially having played on the Wednesday night. Psychologically it"s always that bit more difficult when you've been through a tough night.
"It was just good to finally get the three points."
McManus netted his last goal before Sunday's effort for former club Bristol City in a 4-2 win over Peterborough last year, two days before New Year's Day.
But he has already put through his own net twice since signing for the Fir Park side and was relieved to finally find the target at the right end against the Leith men.
"I think my last goal was for Bristol City - whenever it was, it was too long ago," he joked.
"I've managed to score a couple in my career. It was a great ball in to be fair from Lionel Ainsworth's corner. I was just trying to time my run in.
"When you've got a ball in of that quality it makes my job a bit easier.
"I've managed to put it into my own net a couple of times this season so I'm glad to score at the right end."