Blair was injured in the second quarter of the Seahawks' win over the New England Patriots on Sunday when his right knee was hit hard by teammate K.J. Wright, causing it to bend awkwardly. Irvin went down late in the fourth quarter with what the Seahawks (2-0) initially thought was a sprained knee, per Carroll.
"Both those guys have a lot to offer your team in their attitude and their approach and their style of play, and so it'll be difficult to replace those guys in that regard," Carroll said. "... We're going to miss those guys and feel terrible for them."
The Seahawks signed the 32-year-old Irvin to a one-year, $5.5 million deal in March, bringing him back to the team that drafted him 15th overall in 2012. Irvin has been Seattle's starting strong-side linebacker and a pass-rushing end in nickel situations.
Blair, last year's second-round pick, got off to a promising start in his second season after playing sparingly as a rookie. He was one of the standouts of training camp, as he made a seamless transition from safety to nickelback.
"One guy at the start of his career, another guy that's been through a lot of stuff in Bruce coming back around to us," Carroll said. "It was really great to have him back. It's such a positive factor and such a great example of what our program is about, and I really loved him on this team. The other side of it is Marquise just breaking in for the first time and getting it rolling and really making a big splash to start the season with us. Guys in different stages of their career, but they're both feeling it, and we're feeling the loss."
Carroll wouldn't share the plan for replacing Irvin, but he said rookie first-round pick Jordyn Brooks has shown in limited action that he's ready to play. Brooks has been on the field for 15 defensive snaps in the first two games while backing up Wright at weak-side linebacker. Cody Barton, a third-round pick last year, is another option.
Ugo Amadi, a fourth-round pick last year, took over for Blair at nickelback on Sunday and will remain in that role. Carroll praised the job he did in relief against New England and said Amadi is "way ahead" of where he was last year.
"So as he comes back to us, we won't have any restrictions on what we can do, and we look forward to his input," Carroll said. "We need him to step up and be a factor. He's filling some big shoes. Marquise really had done a great job to this point, and so we're calling on him. He's going to play a little bit different, a little different style to him, but we're really counting on him to come through."
With 52 career sacks, Irvin is the most accomplished member of a Seahawks pass-rush group that entered the season as one of the team's biggest unknowns after the team did not re-sign Jadeveon Clowney. His season-ending injury comes as the Seahawks are already without another defensive end in Rasheem Green, who didn't play Sunday because of a neck stinger.
Carroll said the statuses of Green and swing tackle Cedric Ogbuehi (pectoral) are uncertain for this week's game against Dallas. He doesn't expect wide receiver Phillip Dorsett to play, saying that Dorsett needs more rest after missing the first two games because of a sore foot.
Left tackle Duane Brown came out of Sunday's game because of a sore foot of his own, Carroll said. Strong safety Jamal Adams and cornerback Tre Flowers each dislocated a finger against New England, but Carroll doesn't expect either to miss time.