"Derek is a guy that I've been around in Carolina," McDermott said. "He brings experience, leadership presence to the table and he's worked hard the last week, week and a half, to get up to speed."
Said Anderson: "It's football. Things that happen in this game never amaze me. Just got to be ready at all times. I was fully aware of what I was getting myself into coming here. Obviously not ideal, but we're going to do what we can. ... It's not exactly how I saw it happening, I'm not gonna lie to you."
There is no plan for Allen, the No. 7 overall pick in April's draft, to have surgery after receiving a second opinion from renowned orthopedic surgeon James Andrews, McDermott said.
"The diagnosis was the same as the first opinion," McDermott said. "Everybody is on the same page with what it is and how we need to move forward. We'll be cautious and calculated with that."
Allen said he wants "to be back as fast as possible," but he doesn't know when that'll be as there is no timetable right now.
"I'm trusting the doctors we've talked to and trusting what we've seen," Allen said. "... It's seeing day-to-day, week-to-week how I'm feeling, how we're progressing. Being in the training room like heck and going from there."
McDermott's decision to start Anderson comes after Nathan Peterman, who had served as the Bills' primary backup since being benched in the season opener, threw two interceptions in the closing minutes of Sunday's game.
In seven career appearances, Peterman has thrown 10 interceptions on 82 pass attempts.
"That's always part of the evaluation," McDermott said of Peterman's performances. "But one piece, not every piece of it. I feel like this is the right decision for our football team right now."
McDermott said he did not fear losing the locker room if he chose to start Peterman again.
"I certainly trust and have a heck of a lot of respect for our locker room," McDermott said. "So with that, I'm always going to do what I feel is right for the football team."
"Hindsight is 20/20. It is football," Allen said. "If it had just been Mercilus that hit me, I would be fine. If it was just been [Jadeveon] Clowney [from the opposite direction] that hit me, I would have been fine. They kind of came in together. I saw the replay.
"Whether it was late or not, that's not up to me to decide. I threw the ball. I took the brunt of the hit. That's football. It's a man game. It's a tough game. I still love this game regardless of what happens."
McDermott said he took issue with the hit by Mercilus and has "addressed it with the powers that be" in the NFL.
"I'm not going to go any further than that," McDermott said. "I've learned my lesson from Mike Tomlin."
Tomlin, the Pittsburgh Steelers' coach, was fined $25,000 by the NFL last week for criticizing officials.
Anderson, 35, signed last week with the Bills after spending seven seasons as the Carolina Panthers' backup quarterback. He started seven games during the 2009 season for the Cleveland Browns under offensive coordinator Brian Daboll, who is in his first season as the Bills' offensive coordinator.
"That's part of what factored into the decision in terms of making sure he was able to get up to speed," McDermott said. "If he was a quarterback who hadn't been around the system, it would have been awfully hard to get up to speed in a short amount of time."
McDermott said the team's training staff is comfortable with Anderson's physical condition after he spent all of training camp, the preseason and the first five weeks of the regular season out of football.
Anderson has a 20-27 record in 47 career starts for the Panthers, Browns and Arizona Cardinals. He has completed 53.5 percent of his passes in those 47 starts, with 52 touchdowns, 50 interceptions and a 71.0 passer rating.
"I kind of get to step back and see [Anderson] from a different lens," Allen said. "See how he controls the huddle, see how he operates. I'm looking at it as a blessing in disguise right now to see how he operates, how long he's been in the league, trying to pick things from him."