Rookie receiver Stephen Hill also is out against San Francisco, also with a hamstring injury. Hill strained his right hamstring in New York's 23-20 overtime win at Miami last week.
Keller had been expected to return to the lineup this week after missing two games with a pulled hamstring.
Without Keller, Sanchez has leaned heavily on wide receiver Santonio Holmes. In fact, he targeted Holmes on 25 of his 72 pass attempts over the last two games. In Miami, Holmes tied a career high with nine receptions on 14 targets -- the most times Sanchez has ever thrown to a wide receiver in a game, according to ESPN Stats & Information.
Information from ESPNNewYork.com's Rich Cimini was used in this report.