Both Old Firm rivals go head to head at Hampden Park on Sunday in a massive Scottish Cup semi-final.
Victory for Rangers would deny Brendan Rodgers's side a historic double treble, with Celtic already winning the Scottish League Cup and effectively defending their Premiership title.
Graeme Murty signed four players on loan for the Gers in January, and Bonner has hinted that spending money is the only way that Rangers - 13 points behind Rodgers's troops in the league - will bridge the gap to their Glasgow neighbours.
He told The Glasgow Evening Times: "If you have good organised players you can challenge any team but you have to get the players in, that's the first thing, and you have to spend money.
"You can't hope to have a loan player here, a loan player there and pick the odd one up. You have to spend money to compete against Celtic and that is going to be the big challenge for Rangers, no doubt.
"And if that challenge comes then Brendan Rodgers and Celtic will probably lift their game. They are doing what they have to do to win championships and if they win the treble this year, again, it will be a remarkable achievement.
"Twice in a row, it hasn't been done before so yes, to do it, I think it will take a few years yet."
Bonner's comments could be considered a bit of a dig at the Gers' recruitment policy.
Then again, some of their loan signings - particularly Jamie Murphy - have been outstanding and if the club's Director of Football Mark Allen can sign up the likes of Jason Cummings and Murphy to long-term deals this summer then it'll go a long, long way to competing with Celtic.