This news cannot be considered a surprise. One would expect the market for Reed to be hot and heavy given the hefty price garnered by star relief pitchers at the 2016 trade deadline and the solid returns already seen for the first bullpen pieces moved this season.
Earlier today, the Mariners acquired Marlins setup man David Phelps for a package of four prospects, highlighted by their sixth-ranked prospect, 19-year-old outfielder Branyan Hernandez. Phelps has been impressive out of the pen for the Marlins—and is under team control through 2018—but has not excelled to the extent Reed has.
Last season, the Yankees famously acquired premium prospects by moving Andrew Miller and Aroldis Chapman before the trade deadline. While many may not see Reed in that class, his ERA since becoming a Met on August 31, 2015 has been bettered by only Miller, Kenley Jansen, and Clayton Kershaw. Reed has demonstrated the flexibility to set up and close this season, and should indeed net an impressive return. The Mets’ bullpen should be quite… interesting to watch if and when he is moved.