After losing Javi Martinez to a serious knee injury, the German champions agreed to pay Roma a reported €37 million (£29.4m) to acquire the Morocco international.
However, even though Benatia established himself as one of the best centre-backs in Serie A last season, Robben has now confessed that he is unfamiliar with the 27-year-old defender.
"Honestly, I do not know him," the Netherlands international told reporters. "I only know that he comes from the Roma
"But the club will know why they got him. Javi is a great loss, so we needed a replacement."
Meanwhile, former Chelsea and Real Madrid winger Robben insists that joining Bayern was the key moment in his career.
"For me, it was a very important decision to come to Munich," he mused.
"I was previously at top clubs such as Real Madrid or Chelsea and obviously also had good times there.
"But I've only reached my top level in Munich. I have developed myself over the years as a player and as a person. I am very happy."