Monroe, who apparently sustained a concussion late in last week's win over Cleveland, failed a concussion test Friday and was ruled out for Sunday's game.
"It's out of my hands and in the hands of the doctors," coach Jack Del Rio said. "It came as a shock to all of us. After him not reporting anything Sunday, going through the workout Monday and mentioning he happened to have a headache, it's shocking to see it went this far and that he'd have to be held out. It's not my call."
Del Rio declined to identify Monroe's replacement, but acknowledged it will be a huge concern against defensive ends Osi Umenyiora and Justin Tuck. Right tackle Eben Britton was placed on injured reserve last month. Top backup Jordan Black is filling in for Britton, leaving undrafted rookie Kevin Haslem or inexperienced veteran Guy Whimper as potential options on the other side.
It would be a homecoming for either one. Haslem played at Rutgers, and Whimper spent the last four years with the Giants.
"That was going to be a problem area whether Eugene played or not, so it just becomes a little more of a problem area," Del Rio said. "We know we'll have to help some and felt like we'd have to do that with Eugene if he did make it back. It's going to require some attention."
Monroe missed all three days of practice this week.
Running back Maurice Jones-Drew (abdomen) and receiver Mike Sims-Walker (ankle) were limited in practice Friday and listed as questionable. Barring any setbacks Saturday, they should be able to play.
Copyright 2010 by The Associated Press