Celtic won an unprecedented quadruple treble on Sunday when they beat Hearts on penalties to win the Scottish Cup at Hampden Park.
The Bhoys led to 2-0 and looked set to win the cup before Hearts scored two to level things up, and both sides scored once each in extra time to take the game to a penalty shootout.
McManus accepts that Celtic fans should be delighted by the achievement of their club in winning 12 consecutive domestic trophies, but feels the performance in the final raises questions over whether they will lift any more silverware this season.
He slammed the shambolic nature of their defending and conceded that it still remains a major area of concern as Celtic move into the second half of the campaign.
McManus said on PLZ Soccer: “You are delighted if you are a Celtic fan as you are watching that game and you win 12 consecutive trophies.
“It is an unbelievable achievement, but still in the back of your mind is a big worry about the season ahead in terms of the rest of the trophies.
“Defensively again Celtic were awful; the centre back Shane Duffy and the goalkeeper Conor Hazard were poor, he redeemed himself in the penalty shootout.
“I still think there are massive questions about that Celtic defence.”
Celtic will next be in league action on Wednesday evening when they take on Ross County at Parkhead.
