Since signing the agreement committing him to the club till 2023, the forward has scored just once in six matches and displayed a notable dip in form.
“There has been a drop-off," Bent said to talkSPORT.
“You look at his form in the semi-final and the final of the FA Cup last season, the Community Shield and his performance [in the season opener] against Fulham – he was arguably one of the best strikers in the Premier League, probably THE best.
“But since he signed that new contract, it might be a coincidence, but he's looked a shadow of himself.
“He's missed big opportunities in games, he's not working with the same intensity, he doesn't look like he's enjoying it as much."
Bent continued: "Listen, confidence is massive in a striker. It is with any player, but particularly with a centre-forward, when you're used to scoring as many goals as he is and then you suddenly stop scoring, it can be a little bit of a concern.
“But I think it's even the workrate as well.
“You remember, he was putting shifts up and down, he was all over the place, but he's been nowhere near that level recently."