Wenger surprised by Bentley slump
The former Blackburn Rovers wide-man enjoyed a healthy pre-season under Juande Ramos at the beginning of last season following a £15 million move, before scoring a stunning 40-yard goal in the North London derby against the Gunners 12 month's ago.
However, the 25-year-old subsequently struggled to establish himself in new manager Harry Redknapp's first-team plans in 2008/09.
The England international has recently returned to the Tottenham starting XI due to an injury to Aaron Lennon, but failed to make an impact in Saturday's derby defeat at the Emirates Stadium.
And Arsenal supremo Wenger - who allowed Bentley to leave Highbury for Blackburn in 2006 - revealed he was unsure why the winger's career has stalled since joining Spurs, but insisted the player has always maintained a positive attitude.
"On the day [last season's 4-4 draw] he played well. Then he disappeared slowly last year when Redknapp came in," he told the club's official website.
"It's very difficult for me to speak about that because I don't really know what has happened.
"I had a very young Bentley, he was not completely in our first-team squad before he left here.
"But he was always a guy who had belief in his ability. It was always one of his strengths.
"Bentley responded well when he went to Blackburn because he made a very good career there and was a very influential player."