The Raiders will be without their second-round pick for the first eight games, at least.

Dallas Cowboys' Alfred Morris (46) is tackled after a run by Oakland Raiders safety Obi Melifonwu (20) in the second half of a preseason NFL football game, Saturday, Aug. 26, 2017, in Arlington, Texas. (AP Photo/Michael Ainsworth)

Raiders second-round draft pick Obi Melifonwu answers questions from the media during rookie minicamp on Friday, May 5, 2017, at Oakland Raiders Headquarters, in Alameda, Calif. Benjamin Hager Las Vegas Review-Journal @benjaminhphoto

ALAMEDA, Calif. — An ankle injury forced Raiders safety Obi Melifonwu to miss a chunk of training camp.

A knee injury will cost him at least half of the regular season.

The rookie second-round pick underwent knee surgery last week, sources said. He and guard-tackle Denver Kirkland were placed Tuesday on injured reserve. Linebacker Xavier-Woodson Luster and safety Erik Harris were signed in corresponding moves.

Melifonwu and Kirkland are eligible to return this season. Neither can be activated from injured reserve before Week 9 when the team faces the Miami Dolphins on Nov. 5.

It is unclear exactly when Melifonwu was injured, although he played in one exhibition on Aug. 26 against the Dallas Cowboys. He did not practice on Aug. 29 and wasn’t present at the Oakland-Alameda Coliseum when the Raiders concluded their preseason on Aug. 31 against the Seattle Seahawks.

Tuesday’s signing of Harris was urgent.

Before the move, the Raiders were down to three healthy safeties for Sunday’s season opener versus the Tennessee Titans. Shalom Luani, a rookie seventh-round pick, was the lone reserve behind starters Karl Joseph and Reggie Nelson. Safety Keith McGill II remains on the 53-man roster, but he suffered a Jones fracture during an Aug. 5 practice and underwent foot surgery later that month, a source said. The team expects McGill to miss fewer than eight games, hence why he’s still on the active roster.

Kirkland suffered a similar injury as McGill. On Aug. 12, a Raiders offensive lineman pushed Arizona Cardinals linebacker Alex Bazzie to the ground during the exhibition opener.

Bazzie landed awkwardly on Kirkland’s right foot, causing a fracture of the fifth metatarsal, a source said. Kirkland underwent surgery and has not practiced since.

Starting in 2017, NFL teams can activate up to two players from injured reserve in a given year. There is no guarantee Melifonwu and Kirkland both will be the team’s choices for activation.

Twenty-five defenders currently are on the Raiders’ Week 1 roster. Of them, Harris became the first non-rookie free agent that Oakland has signed since safety Karl Joseph on April 7, 2016. He spent the 2016 season with the New Orleans Saints following a three-year tenure with the Hamilton Tiger-Cats in the CFL. He appeared in four games last year but was waived Friday as part of the Saints’ cutdown from a 90- to a 53-man roster.

Woodson-Luster is back after a brief break, having been waived on Saturday. The undrafted rookie from Arkansas State provides needed depth at a shallow-stocked position. He also can contribute on special teams.

Notable

■ Raiders kicker Sebastian Janikowski remained on the roster Tuesday at 1 p.m. With that, $238,235 of his $4.05 million salary became fully guaranteed. If he is on the roster Saturday at 1 p.m. at his current value, he will collect the other $3,811.765. ESPN reported Monday that Janikowski has declined a pay cut from the Raiders. He already is reportedly dealing with a back injury. The team worked out kickers Josh Lambo, Mike Nugent and Marshall Koehn on Monday. Janikowski declined an interview Tuesday in the locker room.

■ Cornerback David Amerson passed the league’s concussion protocol. He’s slated to start on Sunday.

■ Now that Nicholas Morrow has made the team, he has a new number. The linebacker was assigned No. 50 after wearing No. 35 all offseason. Morrow is an undrafted rookie from Greenville College, a Div. III school in Illinois.

Contact reporter Michael Gehlken at mgehlken@reviewjournal.com. Follow @GehlkenNFL on Twitter.