The 49ers lost a third inside linebacker to a season-ending injury.

Reserve Shayne Skov, who left Sunday’s loss to the Dolphins near the end of the first half, will go on season-ending injured reserve, San Francisco announced Tuesday.

He joins NaVorro Bowman (Achilles) and Ray-Ray Armstrong (pectoral) shelved for 2016 after sustaining their injuries Week 4 and Week 2, respectively.

Safety Marcus Ball has been promoted to the active roster from the practice squad in a corresponding move. Ball was added to the practice squad last week after Vinnie Sunseri was promoted after Eric Reid was ruled out for the year with a torn biceps tendon.

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Skov, in scant playing time, registered two tackles on defense while getting the majority of his run on special teams. The team’s remaining inside linebackers are Nick Bellore, Michael Wilhoite and Gerald Hodges. That trio has spent the bulk of the season platooning in coordinator Jim O’Neil’s 3-4 defense the absence of Bowman and Armstrong.

San Francisco heads into its next game in Chicago (2-9) with the NFL’s last-ranked defense in yardage and scoring. The 49ers were the first team in league history to allow 100-yard rushers in seven straight games from Sept. 18 to Nov. 6 and rank 32nd allowing 172 yards rushing per game.

Linebacker Wynton McManis was signed to the practice squad. He initially joined the 49ers as an undrafted free agent in the spring and was among first cuts in August.