O'Brien was unavailable to pitch in Sunday's extra-inning loss to the Reds due to illness, John Denton of Roundtable Sports reports.
It provides an explanation as to why O'Brien didn't appear in the ninth inning when the game was knotted up. The Cardinals closer should be considered day-to-day. If a save chance arises Monday versus the Cubs and O'Brien remains unavailable, George Soriano or Ryne Stanek could be used.