This news comes after reports that the team is placing veteran cornerback Richard Sherman on short-term injured reserve, and Ahkello Witherspoon is in the concussion protocol.
The 49ers are preparing for their Week 2 matchup at MetLife Stadium against the New York Jets.
Allen was among four cornerbacks to work out for the 49ers on Tuesday. The others were Tino Ellis, Marquis Bridges, and Christian Angulo, per the Houston Chronicle.
The Steelers made Allen a fifth-round draft pick in 2017. He landed on injured reserve and was waived with an injury settlement before the start of the 2019 season.
Allen spent some time with the Seattle Seahawks last season and had a brief stint before this season with the Buffalo Bills. He has appeared in 16 games over his NFL career and recorded three tackles while playing primarily on special teams.
Johnson was promoted from the practice squad leading to the team's Week 1 game against the Arizona Cardinals but then reverted back following the loss.
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