The former Ibrox star was not impressed with that side of Rangers' game at the weekend.
The Ibrox outfit progressed comfortably to the Hampden date with their arch-rivals on Sunday, winning 3-0 against Hearts.
In the opposition half they commanded the game, getting their full-backs forward and providing in form players like Alfredo Morelos with the ammunition to fire home.
However, Ferguson wasn't completely happy with their performance, feeling that they were not convincing up against the physicality of Hearts' forwards.
The back four is now his big worry heading towards the Celtic fixture.
Speaking to PLZ Soccer, the former Rangers captain said: "The only thing that worries me about Rangers is defensively, the back four.
"I think the goalkeeper’s no issue – I think he’s the best in the league – it’s just the back four.
"Again, there were some instances in the game against Hearts where they were a bit slack, they’ve just got to improve."
Ferguson does go on to say he thinks that the Ibrox side have a great chance of lifting the trophy at Hampden, but can they stand up to the attacking onslaught that Celtic might turn up with?
It should be stressed that Rangers did keep a clean sheet at Hampden on Sunday and, alongside the Hoops, have the best defensive record in the Scottish Premiership.
Both teams have only conceded seven goals in 11 league matches.
So while Ferguson may have issues with the defensive personnel at Rangers, it's fair to say they're coping pretty well this season.
However, Celtic did get the better of Gerrard's side at Ibrox in September and have won their last 30 domestic cup ties.
That record will make them favourites when the final rolls around, so the Gers defence will have to play at their absolute peak if they hope to win medals.
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