Stuart McCall's side ended a 10-match run without a victory in cup competitions, including European games, when they won 2-1 at Livingston on Wednesday night to set up a League Cup quarter-final clash against Aberdeen at Fir Park.
After Morton accounted for McManus's former club, Celtic, the bookmakers made Motherwell and Aberdeen joint favourites to lift the trophy.
But the Scotland defender feels the other clubs in the Scottish Premiership top six, all of whom other than the Parkhead men are in the last eight, are growing in strength so emulating their league runners-up spot or winning a trophy will require a major effort.
McManus, whose side host Ross County on Saturday, told Press Association Sport: "I have grown up saying you want to win every competition that you enter. But you have to be realistic as well, Celtic are going to run away with the league.
"If you can win a cup here, it would be a massive achievement. The last time they won one was '91. So that would be brilliant.
"But we are just coming to work every day and listening to what the manager is telling us.
"The manager has been really good, he has really impressed me.
"If we can build on last season, then great. If we can mirror that it would be an unbelievable achievement because there are stronger teams this year than last year.
"Inverness are looking good, Aberdeen are looking good, Dundee United have started to look a bit stronger and Hibs have kicked on a bit.
"It's going to be a tough season but we certainly feel we are as good as anyone in the league outwith Celtic."
- Sports & Recreation
- Scottish Premiership