Liverpool look likely to offload the big Belgian striker.
The 25-year-old was a £32 million signing under Brendan Rodgers but the Northern Irishman's sacking in October has seen Benteke fall out of favour with his successor.
The former Aston Villa man has also not yet featured for Liverpool in pre-season after falling way down the pecking order, behind Daniel Sturridge, Danny Ings, Divock Origi, Sadio Mané and Roberto Firmino.
His agent confirmed to The Guardian this week that the Belgian striker is the subject of interest, with West Ham, Everton and Crystal Palace among his reported suitors.
And Mariner is confident that he will be sold by September, noting that Klopp "does not want him around".
Asked by ESPN if Benteke will still be a Liverpool player when the window shuts, Mariner said: "I like Benteke but he doesn't seem to fit into Klopp's plans. It's fairly obvious he'll leave Anfield so it's just a question of who'll pay the money. I say that he goes because Klopp doesn't want him around."
Liverpool kick off their Premier League season with a visit to Arsenal on Sunday and it would be a big surprise if Benteke is anywhere near the first-team squad that day.