He wasn't actually bad for a prolonged period in the majors before the two years, just an unhyped prospect at an advanced age. You don't have to buy him being a 'changed' player necessarily, just a player who is different from his prospect scouting reports. And very many players are different from their prospect scouting reports.
Offer him a seven-year deal with a club option tacked on. 6, 8, 10, 15, 16, 16, 18, (20, 3 buyout)