And he called for Brownlow medallist Simon Black to be used as a substitute as he manages his return from a knee injury.
The Brisbane Lions captain's six appearances this season have netted just 34 marks (six contested) and 10 goals, including five against Gold Coast and three against Melbourne.
Walls, a former coach of the Brisbane Bears, fears the 31-year-old's playing days are numbered because he is going to ground too often.
"I really feel as if Jonathan Brown could be coming to the end," he told Melbourne radio station SEN on Monday morning.
"The big fella's been fantastic. He gets knocked around, he's taken a lot of hits, got hit in the head again yesterday (in the Lions' 60-point loss to the Sydney Swans at the SCG).
"But most times he goes for a contest he ends up on his back. And that is, to me, a sign that it takes such an effort to get there that you fall over.
"I think big Jonathan is heading towards the end."
Walls also questioned the Lions' management of fellow veteran Black, who has played in the reserves since recovering from a knee injury.
"If Simon Black's ready to play football, I'd have him as sub," he said.
"I'd get 15-30 minutes of football out of him in the senior team because that's what Brisbane need at the moment."
Walls predicted a gloomy outlook for the Lions, whose next seven opponents are West Coast (home), Essendon (away), Carlton (home), Collingwood (home), Fremantle (away), Geelong (home) and Hawthorn (away).
"They've got some real problems," he said.
"They could lose their next six or seven games. They could be two wins and 11 losses around the half-way mark of the year."
Ben Collins is a reporter for AFL Media. Follow him on Twitter: @AFL_BenCollins
