The England international has enjoyed significant success at Stamford Bridge since departing Arsenal in 2006, winning the Premier League and Champions League as well as the FA Cup on four occasions.
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"Yes, it is a surprise to me that Ashley has been left out by Chelsea," Wenger is quoted as saying by the Daily Mirror.
"I still see Ashley sometimes in big games; he can produce great performances still.
"I don't know whether he is still England's best left-back. My name is not Roy Hodgson."
Arsenal take on Chelsea on Monday, with Wenger conceding Mesut Ozil will require a rest at some stage having been accustomed to winter breaks in Germany and Spain.
Ozil will feature against Mourinho's men as Arsenal seek to move back to the top of the Premier League table.
"[Ozil] wouldn't be used to playing at this time of year," Wenger added. "In Germany they have a big Christmas break, and also in Spain.
"At some stage, I will have to give him a break, but not on Monday night. Ozil has played a lot, but we've had a little breather and, hopefully, he has now recovered."