Former Celtic boss Martin O'Neill insists there's no guarantee Aaron Ramsey will be a Rangers smash hit.
And he's praised the "terrific" job Ange Postecoglou is doing ahead of the Aussie's first home derby.
The Parkhead side have a massive opportunity to go top of the Premiership with a win over their great rivals on Wednesday night.
And deadline day Ibrox signing Ramsey, on loan from Juventus, is highly unlikely to feature given his lack of match fitness.
O'Neill dismissed the idea that the low percentage of Ramsey's wages being covered by Rangers makes the move unfair, responding to an accusation by shock jock Adrian Durham.
"When has anything ever been fair in football?" he said on TalkSport.
"It's the nature of the game and if you get an opportunity to do it, why not?"
But O'Neill, who managed the likes of John Hartson and Chris Sutton at Celtic, warned that Ramsey might take a while to adjust to Scottish football.
He added: "The Rangers side that I was up against when I went to Celtic was remarkable.
"You had De Boer, Van Bronckhorst, Michael Mols. They were terrific footballers at that stage.
"By the time I got there they had well and truly bedded in. But some players we had didn't find it immediately easy.
"They had to adjust to the game, very much so."
O'Neill also reckons Postecoglou has had a remarkable turnaround as he praised the boss' "personality".
"Very impressive," he remarked. "He came in at the start and people were wondering who this character was.
"He took a little bit of time to bed in and results weren't going fantastically well. But he's turned all of that around.
"His personality is terrific and the crowd have really taken to him. It's been a terrific effort by him."
