The number became available because wide receiver Ricky Pearsall is reportedly switching to No. 1, previously worn by Deebo Samuel. San Francisco traded Samuel to the Washington Commanders in exchange for a fifth-round draft pick.
The 49ers selected Pearsall, a Florida product, with the 31st overall pick in last year's draft. As a rookie, he appeared in 11 games with four starts, recording 31 receptions for 400 yards and three touchdowns.
Mordecai, an undrafted free agent out of Wisconsin, spent last season on the practice squad. He previously shared No. 4 with kicker Jake Moody, which wasn't a conflict due to his practice squad status.
H/t to Wayne Brown for the find.