That’s the verdict of former Leeds defender Danny Mills.
Cahill is a free agent having left Chelsea after seven years at the end of last season.
The 33-year-old is keen to carry on playing but is yet to find a new club.
Arsenal and Manchester United are both desperate for defensive reinforcements ahead of the new campaign.
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The Gunners were regularly criticised for their leaky backline last term and club captain Laurent Koscielny is expected to leave after refusing to go on their pre-season tour of America.
Meanwhile, United are prepared to splash out £75m on Leicester defender Harry Maguire.
But Mills has urged the pair to cut costs by snapping up Cahill.
“Obviously he wasn’t quite as good or quite have the presence of John Terry,” he told talkSPORT.
“It’s a no-brainer to take him.”
Former Leeds defender Danny Mills
“But he’s not too far behind that.
“And you think if we want somebody to come in to play two thirds of the games next season.
“But everything that he brings about him, that professionalism, that experience, how players can learn from him on and off the pitch.
“All those types of things that he can add to the football club.
“And he’ll just want to play and he’ll want to be part of it.
“It’s a no-brainer to take him.
“Why the likes of Arsenal, even why are Manchester United not interested?
“They’re desperate for a centre-half.”