The Real Madrid star continues to be linked with a return to Old Trafford this summer and if he does happen to come back, Keown believes his arrival may be the only one new Red Devils boss David Moyes needs to continue on from Sir Alex Ferguson's success.
"It would be an incredible coup. It would make a huge statement and it is exactly what Manchester City failed to do when they won the Premier League last season," Keown wrote in a column for the Daily Mail.
"I would be surprised if he is joined by a raft of new signings at Old Trafford. David Moyes will be a fan of 90 per cent of the players at United because they are top-drawer and he will want to give people a chance to impress him.
"Too many changes can destabilise a club and losing the manager is the biggest change of all. Moyes is cautious and a team builder so Ronaldo would be perfect but he doesn't have to be the start of a summer of madness."