That could pose a serious problem Sunday in the regular-season finale when Manning will attempt to break the NFL's single-season passing record.
Seldom-used Chimdi Chekwa and Phillip Adams will start if Jenkins and Porter are unable to play. Chekwa, a fourth-round pick in 2011, has just one start in his career. Adams has made three and has played mostly as a nickel back for the past six weeks.
''It's going to be a challenge, obviously,'' Chekwa said. ''(Manning) is a great quarterback who knows how to put the ball where it needs to be, so as a defensive back you have to have good technique (and) in position to try to make a play on the ball.''
The Raiders didn't have much success at all against Manning when the two teams met in late September. The Broncos' quarterback completed 32 of 37 throws for 374 yards, three touchdowns and a passer rating of 135.8.
That was when Oakland had a full complement of cornerbacks. Much has changed since then.
If Jenkins and Porter are unable to play Sunday, the Raiders will be down to just three healthy cornerbacks - Chekwa, Adams and Chance Casey, who was activated from the practice squad on Dec. 7. Taiwan Jones is also on the roster but has spent the past month taking snaps as a backup running back.
Chekwa, who had seven tackles in last week's loss to San Diego when Jenkins got hurt, has made just one other start in the NFL - on Oct. 2, 2011, against the New England Patriots and Tom Brady.
Now he'll be squaring off against Manning, whose career he watched closely while growing up in Louisiana.
''I followed Peyton Manning when he was in high school and I was just a kid back in New Orleans,'' Chekwa said. ''To get an opportunity to play against him is exciting. He's going to find the weakness in the defense and we just have make sure we execute the call.''
The Raiders also are trying to snap a five-game losing streak and avoid finishing 4-12 for the second consecutive year.
It won't be an easy task.
Denver has won four straight games by an average of 18 1/2 points in the series between the two AFC West rivals. The Broncos are well-balanced on offense and have a trio of receivers and one tight end who have scored at least 10 touchdowns apiece.
The key, however, remains Manning.
''If you're going to zero in on anybody, you're going to zero in on Peyton Manning,'' Chekwa said. ''You know, try to affect the quarterback. I think that's the only hope of making the play and getting the win.''
Jenkins has battled a variety of nagging injuries throughout the year, but Porter had been relatively healthy until showing up on the Raiders' injury report Friday.
Oakland's need to find healthy cornerbacks coincided with the retirement of former Raiders defensive back Nnamdi Asomugha. A press conference for the three-time Pro Bowl pick was held at the team's headquarters two hours after practice ended.
As much as the Raiders could use the help, however, it appears they're willing to go with Chekwa and Adams if the two starters are unavailable.
''I thought (Chekwa) played pretty well in the last game,'' Oakland coach Dennis Allen said. ''So if Mike's not able to go for whatever reason, Chimdi will go in.''
Notes: RG Mike Brisiel and backup RB Jeremy Stewart are listed as probable after being limited in practice earlier in the week. ... Jones will continue to return kickoffs, according to Allen.
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