SEATTLE -- The Mets completed step one of what they hope is a rapid retooling of the roster, dealing away closer Addison Reed to the Red Sox on Monday in exchange for a package of three pitching prospects, a source confirmed. Only a review of medical records stands in the way of making the deal official before the 4 p.m. nonwaiver trade deadline, a critical period as general manager Sandy Alderson attempts to get the Mets back into contention next year after a season of unmet expectations. In exchange for one of the league’s top relievers, the Mets received righthanded pitching prospects Jamie Callahan, Gerson Bautista and Stephen Nogosek. All three rank in the Rex Sox’s top 30 prospects, according to MLB.com. Reed, 28, enjoyed a career renaissance during his tenure with the Mets, who acquired him from the Diamondbacks in an August 2015 waiver trade for a pair of relief pitchers. At the time, the acquisition had been somewhat of an afterthought. Though he had closing experience earlier in his career with the White Sox and Diamondbacks, he had since bumped back into middle relief. Before his trade from Arizona, the righthander had even spent a portion of the season in the minors. However, upon his arrival in New York, Reed flourished, posting a 1.17 ERA in 17 appearances. By the postseason, Reed had become the primary setup man for closer Jeurys Familia, forcing the Mets to alter their long-term plans. When Reed first was acquired, the Mets had leaned toward non-tendering him at season’s end, a concession to his projected jump in salary due to arbitration. However, he pitched well enough to earn $5.3 million in his second year of arbitration, which proved to be a steal. In 2016, Reed delivered one of the best seasons by any reliever in the National League with a 1.97 ERA over 77 2/3 innings in 80 appearances. This season, Reed spent most of the year in the closer’s role for Familia, whose season has been shortened by a domestic violence suspension and surgery to repair a blood clot. In 48 games, Reed has a 2.57 ERA with 19 saves in 21 chances. However, the Mets’ disappointing season only cemented Reed’s spot on the trading block, especially since he’s due to be a free agent at season’s end. Once Alderson declared the Mets would be selling off their pieces, Reed became the most valuable trade chip, standing out even in a relatively crowded market for relievers. “I’ve been traded twice before,” Reed said as the deadline loomed. “If it’s going to happen, it’s going to happen. There’s nothing I can do about it. So I’m going to come to the field and if my nameplate’s up here, I’m going to suit up and go. And if it’s not, I’m going to go somewhere else.” That somewhere else is Boston. After swinging major deals in each of the last two trade deadlines to acquire Yoenis Cespedes and Jay Bruce, Reed’s departure capped a week in which the Mets found themselves as sellers. It was a fate they had not anticipated after beginning the season with high expectations. Last Thursday, the Mets traded away longtime slugging first baseman Lucas Duda to the Rays for Drew Smith, a hard-throwing Double-A pitching prospect. And on Friday, the Mets made an unusual move, traded a pair of prospects to the Marlins for closer AJ Ramos. The deal was made primarily to bolster the bullpen for 2018. Ramos is under team control for next season, when he’ll be expected to help fill the void in the bullpen left by Reed’s departure.