OAKLAND — Lane Kiffin had concerns about depth on the Oakland Raiders even before two key offensive players went down with season-ending knee injuries.

Those worries have only been heightened now that receiver Drew Carter and fullback Oren O’Neal went down with injuries early in Saturday night’s 24-0 loss to Arizona.

“Obviously, both were really down, especially Drew,” Kiffin said in a conference call Sunday. “He’s had so many things like this happen to him. I believe this might be his fourth ACL maybe in his career. He’s had a lot of tough breaks injury-wise so he was on a one-year contract so this was very tough for him.”

It also was a tough break for the Raiders, who were counting on Carter to perform after signing him to a $2 million contract in the offseason. He set career highs last season with 38 catches for 517 yards and four touchdowns for Carolina last season and was one of only three experienced receivers on Oakland’s roster.

Now the Raiders will need to look for a veteran to complement Javon Walker and Ronald Curry. Kiffin said they could go after an out-of-work veteran or wait until other teams make their cuts this week to find someone to replace Carter. Two possible options are Joe Horn, released last week by Atlanta, and Koren Robinson, who remains unsigned.

The Raiders will need to do something because their entire backup group at wide receiver has just seven career catches. Johnnie Lee Higgins had six receptions as a rookie, and Chris McFoy has one career catch.

“There’s not a lot of history there as far as guys playing in the league besides Javon and Ronald,” Kiffin said. “Really, none of those guys have really played before. So it would be tough to only go with two guys who have played before.”

The injury could open the door for both Todd Watkins and rookie Chaz Schilens to make the roster.

The Raiders are also thin at fullback, where O’Neal was expected to team with Justin Griffith this season.

Now the only other fullback on the roster is Marcel Reece, who played receiver last year in college at Washington.

“That’s a big deal right there,” Kiffin said. “There are a number of games last year he played even more for us than Justin. He’s a guy when we run the ball he is in there play after play, a physical attitude.”