The 34-year-old agreed to a one-year deal with Seattle in late January. While Byrnes is earning $11MM in the final year of the three-year pact he signed with the D'Backs, the Mariners are responsible for just $400K of that sum.
Seattle has also made a trio of in-house adjustments, sending Matt Tuiasosopo to Triple-A while promoting Ryan Langerhans and Josh Wilson. All four roster moves have been confirmed by a team press release.
Shannon Drayer of ESPN Radio Seattle (via Twitter) points out that Langerhans and Wilson were not on the 40-man roster, meaning that another move is on the horizon. She later tweeted that the move will not be shifting a player to the 60-day DL and it will likely not involve a player on the 25-man roster (also via Twitter).