The Newcastle United winger is expected to leave the club.
The 26-year-old winger has been linked to an array of clubs this summer after starring for France at Euro 2016, helping les Bleus finish runners-up in the competition.
Mariner admits he is baffled as to why the Sissoko which dazzled at Euro 2016 was not so prominent for Newcastle, saying his energy during the competition was "phenomenal", but he belives the winger will leave.
"Where was the form at Newcastle that he showed for France?," Mariner said to ESPN. "Yes he was playing with better players but the energy levels he brought to the Euros was phenomenal.
"I can't see him staying at Newcastle. I don't think Rafa's going to want him around."
Newcastle lost 1-0 at Fulham on Friday in their first Championship game of the season and Benitez will want his squad finalised as soon as possible.
Sissoko was absent that day and although the defeat was hardly his fault, the uncertainty over his future certainly did not help matters - and won't help going forward.