Kimbrel was left in the bullpen despite a save situation arising in Sunday's 3-2 extra-inning win over the Rays.
Until we hear otherwise, this appears to be a decision based on Kimbrel's usage over the last week, not an injury. Boston's closer had pitched four of the last five days, so he was allowed to sit this opportunity out despite the Red Sox having an off day Monday. Instead, manager John Farrell left Joe Kelly (2.2 innings) in the game to finish off the Rays in the 10th.