The Raiders waived defensive back Ron Parker on Saturday to make room on the roster.
Janikowski missed practice Thursday and Friday and is officially listed as questionable and coach Hue Jackson was concerned enough to sign Rayner.
Rayner kicked for six teams in five years, most recently for Detroit in 2010. He has made 55 of 75 field goal attempts in his career.
Janikowski has hit 12 of 13 field goal attempts this season. Five of his kicks have been 50 yards or longer, including a record-tying 63-yarder in the opener at Denver.