The 49ers signed Knowles on Wednesday.
Knowles will return to San Francisco after spending time with the team earlier in the summer and then being waived. He will provide the team some much-needed depth at receiver, as Brandon Aiyuk (knee), Jauan Jennings (calf), Jacob Cowing (hamstring) and Jordan Watkins (ankle) are all dealing with injuries in addition to Demarcus Robinson being suspended for the first three games of the season.