Mika Amonsen will replace Andrew Terraciano, who played Sean Reagan throughout the 14-season run of "Blue Bloods."Though Donnie Wahlberg is the only member of the core Blue Bloods cast set to return for the long-running series' first spinoff,

Sean Reagan will be accompanying his father, Danny, to Boston in "Boston Blue," but the series has recast the "Blue Bloods" spin-off's role - here's why.