The midfielder was voted into the PFA League Two team of the season in 2010/2011 season before sealing a move to United worth around £195,000.
Despite showing glimpses of class during his spell at Glanford Park and being likened to Iron legend Lee Hodges, Brian Laws decided not to offer Ryan a new deal.
“We want to replace Jimmy with someone who has a good engine and can go box to box,” said the Iron boss.
“Jimmy is a player I've enjoyed working with, but I've said to him that our offer would have been very low and I wouldn't make that offer because I know he would turn it down.
“Chesterfield are paying him if not equal then more of what he had been getting paid here. We wouldn't have been able to match that.”
Ryan scored 6 goals in his two seasons with United.