The moves were made Wednesday as the Bills (6-9) prepare to close their season at New England (12-3) on Sunday.
The Bills did not disclose the nature of Rayner's injury, though he's been bothered by a sore pelvis. He missed two of six field-goal attempts in a 40-14 win over Denver last weekend.
Rayner signed with Buffalo in early November after Rian Lindell broke a bone in his right shoulder.
Coutu joins the Bills two weeks after being released off the Jacksonville Jaguars' practice squad. He has yet to play an NFL game after being selected in the seventh round by Seattle in the 2008 draft.