Mark Warburton believes Rangers should be patient with Graeme Murty and not expect to overtake Celtic anytime soon.
Warburton, who managed Rangers from 2015 to 2017, said the club need to realise the financial gap with their oldest rivals is widening.
The 55-year-old understood the fans "have suffered" and "crave success", but he urged Rangers fans to manage their expectations.
He told Premier League Daily: "You have to recognise Celtic have been in the Champions League now for the last few years, done well to get into the group stage and with that comes financial rewards.
"All that does there is increase the gap between Celtic and Rangers. Every year you are fighting now to close that financial gap.
"The longer Celtic are exposed to Champions League football, the greater the financial divide becomes.
"Rangers need the investment. They need the investment and need to dampen some expectation and recognise they have to build the club back up.
"It is an outstanding club, it is a football institution, it was an absolute privilege to manage up there but you have got to be patient with it.
"Be honest with the fans and give them communication. They are aspiring to get back to the top of Scottish football but it will take time and it will certainly take some investment."
Rangers are second in the table under Murty but are still 10 points behind the league leaders Celtic.
Warburton believes if the club give Murty the chance to take on the role full-time then they must be patient with him.
He added: "Graeme [Murty] has come in he has earnt the respect of the squad, he has got the fans on side and he has done a very good job.
"If you are going to give Graeme that chance, be patient with him. If the fans think he is going to immediately challenge Celtic next year, if he's not sacked him by Christmas, then you are going to have the same old problems.
"Give the man a chance, give him the time, give him the backing verbally and financially that he needs to manage a club of that stature."