Barcelona have had three bids rejected for the Brazilian playmaker, who has handed in a transfer request at Anfield.
“He [Coutinho] wants out, and if you're getting up to what are they now, almost £120m, it just looks like how much rather than if or when," Barton told talkSPORT.
“He's not a Scouser, he's a Brazilian. He's already living a fair distance away from home and he's probably grown up wanting to play for Barcelona.
"No disrespect to Liverpool, but that's the club Brazilian youngsters grow up wanting to play for nowadays.
"It's just a case of how much and at what moment he goes. Liverpool should have pre-empted this
"They should have known a big club was going to come sniffing for him and, although he's difficult to replace, they should have had replacements lined up for him."