METAIRIE, La. — Linebackers Curtis Lofton and David Hawthorne represented two of the Saints’ most important acquisitions in free agency and quickly took over starting roles in their first preseason with New Orleans.

Now both are injured — Lofton with a high ankle sprain and Hawthorne with torn cartilage in his knee — and it is unclear if either will be ready to return when New Orleans (No. 9 in the AP Pro32) opens the regular season Sept. 9 against Washington.

“We certainly think Curtis is going to be ready for the first game. The diagnosis on David is encouraging,” assistant head coach Joe Vitt said, noting that Hawthorne kept playing in Friday night’s preseason game against Jacksonville even after his injury.

Vitt added that he was happy with the linebackers remaining on the roster, but general manager Mickey Loomis wasn’t about to wait and see if Lofton or Hawthorne would return in time for Week 1. He traded an undisclosed draft pick Monday night to Seattle for a former nemesis, linebacker Barrett Ruud.

Ruud, who is entering his eighth season, started 68 games over his first six season for NFC South Division rival Tampa Bay before moving in free agency last season to Tennessee, and then in April to the Seahawks.

Ruud has 642 tackles, 23 passes defended, seven interceptions, six forced fumbles and five fumble recoveries in his career. “Our pro personnel people do an outstanding job in acquiring talent for us,” Vitt said.

Vitt said Hawthorne was having surgery to repair a meniscus tear on Monday. He declined to say which leg Hawthorne or Lofton hurt.

Jonathan Casillas, who started four games last season, is expected to start for Hawthorne in Friday night’s preseason game against Houston. Before the trade for Ruud, Vitt had indicated that Ramon Humber would fill in for Lofton at middle linebacker.

Humber has been in New Orleans for parts of the past two seasons, and Vitt said the staff has confidence in his ability to relay defensive coordinator Steve Spagnuolo’s calls on the field.

However, Humber must serve a three-game suspension to start the season for a violation of NFL drug policies, meaning he could not fill in if Lofton were to miss the Redskins game.

Chris Chamberlain, another linebacker the Saints signed in free agency because of his versatility and familiarity with Spagnuolo’s system, tore the anterior cruciate ligament in his left knee in Friday night’s game against Jacksonville and is out for the season.

In addition to Ruud, Casillas could be another possible option in the middle because he also has learned the position — even though it has not been his focus — during his three seasons in New Orleans. “If they do call on me to move or alter or expand, whatever it is, I’m always up for the task,” Casillas said. “The character in the linebacker room is pretty high and we can take it in stride and go with it.”

Humber practiced at middle linebacker last year behind Jonathan Vilma, who is currently suspended for the season in connection with the NFL’s bounty investigation. Although Spagnuolo’s defense is new this season, Humber said he was comfortable relaying calls from the coordinator and making sure everyone was lined up right.

“It’s all about trust. I’ve been here before. They know what I can do,” Humber said. “I got my teammates behind me, out there with me and they’re there to help me out and we’re there to help each other. If I have any questions, I ask them. They’ll help me out.”

Humber is heading into his fourth season out of North Dakota State. He began his career with Indianapolis as an undrafted rookie and played against New Orleans in the 2010 Super Bowl.

The Saints then signed him as a free agent during the 2010 season, and he appeared in 13 games in 2011, both at linebacker and on special teams.

“This will be a great opportunity for Ramon Humber to step up,” Vitt said. “He’s been in our program and has played the position. He’ll have a great chance to show his skill level and what he can do.” Notes:

New WR Greg Camarillo, signed over the weekend when the Saints placed Chamberlain on injured reserve, participated in his first practice. The veteran is entering his seventh season after stints with San Diego, Miami and Minnesota.

... WR Adrian Arrington says he still is unsure when he will return from surgery to repair the meniscus in his right knee, but wouldn’t rule out a return by Week 1.

... WR Nick Toon has not yet returned to practice from his left foot injury, but could be seen running sprints behind the end zone on the Saints’ indoor field.

... Vitt says RB Darren Sproles (knee) won’t participate fully in practice or play this week, but Sproles says he would be fine to play right away if it were the regular season. He continues to rest as a precaution.

... RB Mark Ingram, who had offseason surgery on his right knee, didn’t practice on Monday as part of the coaching staff’s plan to give him periodic extra rest.

... Vitt says DE Turk McBride (ankle) is running now and “awful close” to returning.

DT Tom Johnson (right ankle) is expected to be out about two more weeks.