Newcastle United and Sunderland drew 1-1 at the weekend.
Last summer there was talk that Sissoko could move to a top-four club, but he has failed to justify the hype this term.
The powerful midfielder has often looked off the pace in a Newcastle shirt, and Jenas says his performance against Sunderland was poor.
The former Tottenham Hotspur midfielder felt that Sissoko looked lost in the Tyne-Wear derby, and he feels that one of Rafael Benitez’s main tasks will be restoring the French international back to his best.
"One of the biggest disappointments for me in the derby, and for this whole season in fact, was the form of Moussa Sissoko," Jenas said. "Last season he was Newcastle's main man and he was being linked with some very big clubs in the summer. But against Sunderland he just looked totally lost.
"Rafa Benitez needs to find a way to get the Frenchman back to what he was doing last year."
Sissoko started the derby match in a midfield position, but he was moved back to left-back by Benitez towards the latter stages of the contest.
Newcastle did manage to rescue a draw against Sunderland, but Sissoko’s rather anonymous performance was a concern.
The 26-year-old put in an improved display during Benitez’s first game in charge, against Leicester City, but he regressed once again last weekend.
Sissoko has now made 31 appearances for Newcastle this season, but he is yet to score and he has only contributed five assists.