PITTSFORD, N.Y. -- Buffalo Bills rookie linebacker Reggie Ragland will have surgery to repair a torn ACL in his left knee and is expected to miss the entire 2016 season, the team announced Wednesday.

The Bills will now be without their first-round pick, Shaq Lawson, and their second-round pick, Ragland, as they begin the regular season next month. Lawson, who underwent shoulder surgery in May, is expected to return between Week 6 to Week 8, a source told ESPN NFL Insider Adam Schefter.

Despite the losses, coach Rex Ryan isn't lowering the expectations for his defense, which finished 19th in the NFL last season.

"I've said all along that I feel like our defense will be much improved," Ryan said Wednesday. "And I'll stand by that."

Owner Terry Pegula echoed those statements.

"How would you like to be the other guys on this team? You lose one guy and people are saying the season is over? That's crazy," Pegula told WGR 550. "Everybody has injuries. Compared to last year, we're relatively unscathed right now, compared to where we were in camp last year."

Ragland suffered the injury in practice on Friday while pursuing running back Reggie Bush on a play during 11-on-11 drills. Ryan initially said the Bills believed Ragland avoided ligament damage, but he later backed off that statement as the team waited for swelling to go down in Ragland's knee.

"I think any time you have a knee [injury], and when they say, 'Hey, we have major concerns here,' and I told you everything that was told to me, obviously we did have major concerns that he wouldn't be available," Ryan said Wednesday. "We brought in some guys thinking that this was a real possibility."

Bills rookie linebacker Reggie Ragland will have surgery for a torn ACL in his left knee and is expected to miss the 2016 season. AP Photo/Bill Wippert

General manager Doug Whaley said after the draft that Lawson, Ragland and third-round pick Adolphus Washington were all expected to start immediately for Buffalo's defense. Fifth-year veteran Zach Brown has taken over Ragland's role with the first-team defense, while IK Enemkpali has filled in for Lawson during training camp with veteran outside linebacker Manny Lawson also sidelined with a partially torn pectoral. Washington continues to practice with the first team at defensive end.

The Bills also signed veteran inside linebackers Brandon Spikes and David Hawthorne in the wake of Ragland's injury. Both players have run with the second-team defense in practices this week.

"You can play to the strengths of some guys," Ryan said Wednesday. "Zach Brown has got unusual cover skills for a linebacker. You can see him, envision him -- is he an every-down player? I'm not sure. It could be. He's doing a real nice job that way.

"Is a guy like Hawthorne or Spikes or Randell Johnson a possibility? Absolutely. But again, we'll let it play out. We haven't even had a preseason game yet. We'll see what happens as we go."

Buffalo traded up to take Ragland with the No. 41 overall selection in April's draft. Whaley said the team considered selecting the Alabama linebacker with its No. 19 pick in the first round, but the team drafted Shaq Lawson instead.