Manning, who is on the physically unable to perform list, has never missed a game in his 13 previous seasons. He is recovering from surgery to correct a disk-related issue in his neck on May 23.
"We r evaluating the QB sitch,#18 healing but we got 2 b prepared 4 early season possibility without him, defense has 2 pick it up, big time!," Irsay said via Twitter.
Irsay also said the team would not take Terrelle Pryor, who is eligible for the NFL supplemental draft on Monday. Pryor won't be allowed to play or practice for any team that signs him for the first five games of the 2011 season.
"Not taking Pryor...what VET QB would u vote 4 to sign that's out there? If sum1 has 2 hold the fort early season, who u going with?," Irsay tweeted.
Manning, who signed a five-year, $90 million contract last month, has passed for 54,828 yards and 399 touchdowns in 208 consecutive starts. He guided the team to a Super Bowl title after the 2006 season.
The four-time MVP also had surgery in March 2010 to relieve pain in his neck caused by a pinched nerve and did not miss camp. However, he was unable to meet with team medical personnel this spring and summer because of the lockout.