The fullback is set to sign for Roma today from Ajax, having touched down in the capital on Wednesday.
Rensch will move to Roma for €6m, agreeing terms over a contract to 2029.
At yesterday's Europa League preview for their clash with AZ Alkmaar, Ranieri said: "He doesn't have physical strength, but he has magnificent timing when it comes to insertion, good foot both right and left, he has to improve but I like him and I'm sure he will give us great satisfaction."
Ranieri also said of tonight's tie: "I am sensitive because I always want to win. It will be tough, they are a young team that goes at 3000 an hour in transition, we expect a great performance and I am curious to see how we will react.
"The team is doing well, they are motivated, they are doing well, I am sure we will play a great game."