It is the main part of a major shakeup on the series, which also includes the departure of fellow original cast member Tamala Jones. Fillion has been in negotiations to return to Castle, which I hear is eyeing a shortened, likely 13-episode.
TVLine has learned that in the 19th episode of Season 8, Silverman plays Alan Masters, the miraculous survivor of an attempted murder who subsequently helps 城 and Beckett solve the case.