Rangers suffered a disappointing 2-1 defeat at Hampden Park on Sunday as Celtic won the first trophy of the season in the form of the Scottish League Cup.
With a nine-point lead at the top of the league table, Celtic are almost certain to win the Scottish Premiership again this season.
And Ferguson feels now the Scottish Cup becomes the most important tournament of the season for Rangers.
They won the trophy last season and the Rangers legend insisted that they have to retain it in the ongoing campaign.
He conceded that it is hard to see how Rangers can chase Celtic down in the league but feels their complete dominance can be halted by the Gers if they win the Scottish Cup.
The Rangers legend said on the Go Radio Football Show: “Going forward, I know what they need to do. They need to retain the Scottish Cup.
“They can’t give up on the league, albeit maybe it will be tough the claw those nine points back.
“But listen you never know in football.
“They need to do one thing for sure, which is to win the quarter-finals, semi-finals and the final at Hampden and stop the dominance of Celtic this season.”
Rangers will host Raith Rovers in the quarter-finals of the Scottish Cup on Sunday, 12th March.
