The Scottish Championship leaders face their bitter rivals at Hampden Park next month.
The two sides were drawn together in the semi-final of the Scottish Cup in what will be only their second meeting in four years due to the Gers' demotion to the bottom division.
And while Ronny Deila's men are in a good position to secure their fifth Scottish Premiership title in a row, while Rangers are closing in on promotion to the top flight, McLeish suggested that the nature of the upcoming contest means that current stature could well go out the window.
"Celtic are seen as favourites because of the disparity between the two divisions but you just never know," he told the Record.
"Warburton will achieve his goal of getting back to the Premiership and that was his main aim.
"The Old Firm game is a one-off. Anything can happen in this game. I don't think there's a massive difference in the squads. I do believe Rangers are playing with a lot of confidence and will play football that can make the game much closer than last season.
"They will be the underdogs but they might spring a surprise. In an Old Firm game it should never be a shock if either team wins despite one team seemingly having more quality than the other."
The way that Mark Warburton has changed things quite considerably at Ibrox since he arrived means that last season's League Cup semi-final - which the Hoops comfortably won 2-0 - should count for little when they meet again.
The Gers proved in the previous round that they were capable of dismantling teams at the highest level as they thrashed Dundee 4-0, and they will surely be using that result to give them belief that they can beat the Bhoys, even if Celtic are going to be a much tougher opponent.