Ally McCoist's side are aiming to take their second SPL scalp of the season tomorrow night when they entertain Caley Thistle in the Scottish Communities League Cup.
The Ibrox outfit have already eliminated Motherwell from the competition and can secure a semi-final place at Hampden Park if they manage to overcome Inverness.
But nine-in-a-row legend Bomber reckons the Light Blues wouldn't be good enough to mount a top half of the table challenge despite big-name arrivals such as Dean Shiels and Ian Black.
Brown believes McCoist was sold short by owner Charles Green in the summer and has questioned the wisdom in signing Brazilian defender Emilson Cribari.
He said: "I have no doubt that Rangers would struggle over the course of a season in the SPL with the squad Ally has right now at his disposal. I genuinely believe that there is just not enough depth of quality in it.
"What you are seeing with some of the players, such as Cribari, who have come in is that they are just not good enough to help the young guys who are being pitched in develop and learn during the course of the games.
"I question why these guys have been allowed to sign contracts with Rangers when they are plainly not good enough and the buck in that respect stops at the feet of Charles Green.
"Tomorrow night's tie with Inverness is a massive test for Rangers and the support will be watching to see that the victory over Motherwell was not a fluke. But the bottom line is that Ally has not been allowed to bring the quality of player in sufficient numbers in that he needed to."