Southampton-born Roberts had an impressive career as a player, representing clubs suchas Tottenham and Rangers, as well as earning full England honours.
Johnson, 21, is reported to be disappointed at a lack of first-team opportunities, and has attracted the interest of teams suchas Glasgow Rangers and Fulham.
Since arriving in Nashville, Josh Hamilton has been rumored to have met with the Mariners and has been tied to conversations with ball clubs suchas the Rangers, Brewers, Phillies, Red Sox and even the Yankees.