Las Vegas Raiders defensive end Malcolm Koonce of an NFL football game against the Los Angeles Chargers.

Raiders defensive end Malcolm Koonce was placed on injured reserve Saturday because of a knee injury, meaning he will miss at least the first four games of the season, including Las Vegas' Sunday contest at the Los Angeles Chargers.

The club also signed defensive end Charles Snowden to the active roster and activated cornerback Sam Webb from the practice squad.

Defensive ends Janarius Robinson and Tyree Wilson are expected to receive additional playing time in place of Koonce.

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Robinson played in six games and started two last season, totaling eight tackles and a sack. Wilson, the Raiders' No. 7 overall pick last year, played in all 17 games, but didn't start any and finished with 29 tackles and 3 1/2 sacks.

Koonce, one of the Raiders' top pass rushers, was injured in practice Thursday.

He played especially well in the second half of last season in which he made all eight of his career-high sacks beginning Nov. 5 against the New York Giants. Six of his sacks came in the last four games.