BRADENTON, Fla. -- The 49ers re-signed defensive back Antone Exum to a one-year contract on Wednesday, as the club begins preparations to face the New Orleans Saints with some question marks in their secondary.

Starting strong safety Jaquiski Tartt is uncertain for Sunday’s game due to broken ribs that he sustained in the 49ers’ 20-17 loss to the Baltimore Ravens in Week 13.

Also, cornerback Richard Sherman is hobbled with a knee sprain, and his status for the game Sunday is unclear, too.

Exum fills the roster spot that was vacated when the 49ers waived kicker Chase McLaughlin on Tuesday.

Marcell Harris would start in place of Tartt, if he is unable to go. And if Sherman is unavailable, the 49ers will start Ahkello Witherspoon and Emmanuel Moseley at the cornerback positions.

Exum began the season on the 49ers’ 53-man roster and appeared in the first three games of the season. He was released on Oct. 2 when Harris was promoted from the team’s practice squad.

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Exum, 28, appeared in 20 games for the 49ers over the past three seasons with eight starts. He appeared in 26 games in 2014 and ’15 with the Minnesota Vikings.

The 49ers also opened the practice window for defensive lineman Kentavius Street on Tuesday.