Alexander is now on injured reserve and the 49ers announced that they have promoted linebacker Elijah Lee from the practice squad to take his spot on the 53-man roster.
Lee appeared in 30 games for the 49ers over the last two seasons. He has 69 tackles, two forced fumbles and a sack.
The 49ers also announced that they have opened a 21-day practice window for tight end Garrett Celek. Celek had back surgery during the offseason and he was placed on the physically unable to perform list at the start of training camp this summer.