The Lakewood sheriff's station posted a photo of the dictated letter Wednesday on its Facebook page and said Deputy Brandon Love, who remains hospitalized, had also recorded a short video for the unidentified bystander.
Love requested that the video only be shared with the good Samaritan, who he called his "guardian angel," according to sheriff's officials.
"I was told it was highly possible I would never walk again. A few days out, I am making progress and my hopes are high," Love says in the letter. "Thank you all for the support and positive thoughts."
Love remained hospitalized Thursday, but he will be moved to a rehabilitation facility soon and "is promising he will walk out of the hospital," the post said.
Love, a 13-year veteran, responded to a domestic-violence call Aug. 15 at a mall in Lakewood, where a man and woman were fighting. As one deputy interviewed the woman, Love went to search the mall for the suspect, later identified as Frankie Estrada, 21. He found Estrada and was walking him out of the mall when Estrada attacked him, knocked him to the ground and began kicking and stomping on Love's head and neck, authorities said.
A bystander intervened and pulled Estrada away long enough for other deputies to reach Love.
Sheriff's investigators are not releasing the good Samaritan's name because the investigation is ongoing and he would also be considered a witness, said Sgt. Joe Reveles of the Lakewood sheriff's station.
Estrada, of Bell Gardens, pleaded not guilty earlier this week to charges of attempted murder of a peace officer, aggravated mayhem and other charges.