In the summer transfer window of 2008, Mensah was linked with Portsmouth, which also looked likely to be his future destination due to strong presence of African players, along with his international teammate Sulley Muntari at the club. He was also linked with a transfer to Wigan Athletic. An enquiry from Wigan for the Ghanaian was welcomed by Rennes, with assistant coach Philippe Redon commenting it would take at least £7 million to prize the "Rock of Gibraltar" from the club. He was made Rennes' team captain in January 2008 until his exit that summer. Following his departure, Rennes head coach Guy Lacombe lamented he had hoped to keep his captain because his exit will mean a re-examination of their playing system and that, "e will not go to an average club. If John leaves, he will do it only for something that one cannot propose to him here."