Simpson and goalkeeper Dean Bouzanis headed the list of five players released at the end of their contracts on Wednesday and Johnson paid warm tribute to the former Coventry and Huddersfield forward.
He told Latics' official website: "Robbie has been fantastic during the short time I have been here.
"I like him as a person and he gave everything whether in a match or on the training pitch.
"It is a measure of how highly I regard Robbie that I made him captain when James Wesolowski was ruled out.
"And as I have mentioned before, if you were in the trenches, Robbie is the sort of character you would like alongside you."