JACKSONVILLE, Fla. — Just when the Jacksonville Jaguars thought they had a defense that could set the tone for the team, they learned they lost two key members of that unit.

Middle linebacker Paul Posluszny tore a pectoral muscle in the Jaguars’ 24-6 victory over the Cleveland Browns on Sunday, and he will undergo season-ending surgery. Backup defensive Andre Branch will be out six weeks with a torn groin muscle.

Chris Smith, a rookie defensive end, was activated from the practice squad to take the roster spot of Posluszny, who went on injured reserve.

The injuries occurred at a time when the Jaguars’ defense, after giving up 15 touchdowns in the first four games, limited the past three foes to just two touchdowns.

The Jaguars, who are thin at middle linebacker, will replace Posluszny with inexperienced J.T. Thomas.

“Well, it’s a big loss,” coach Gus Bradley said of Posluszny. “He’s really been playing well ever since I’ve been here, but I think he’s come into his own, really understands the defense now and really takes total command of the defense. That’ll be a difficult one.”

Posluszny is noted mainly for his play against the run. He is the league’s leading tackler since 2008 dating back to his days with the Buffalo Bills. He sometimes struggles in pass defense, but the Jaguars don’t have a better pass-coverage linebacker to replace him.

His mere presence will be difficult to replace.

“Everybody has different leadership style,” Bradley said. “Poz was not really a vocal-type leader. He led by his actions and his demeanor and how he took care of himself, so hopefully the lessons he’s taught many of our players, they’ll buy into and continue to own it and they’ll demonstrate it. I think it’ll be a group effort.”

Posluszny was injured sacking Browns quarterback Brian Hoyer midway through the fourth quarter. He stayed in the game for a play, then he came out after Hoyer’s incomplete pass on third down.

On the sideline, the doctors figured it was a serious injury, and an MRI exam confirmed it.

“I knew something was wrong (when it happened), but not that wrong, you know what I mean,” he said. “We’re winning, we’re up, you want to finish the game.

“It’s not easy, especially (because) you get the feeling now that we’re just starting to pick things up and the team’s going to roll a little bit and get on a nice winning streak and play some sound football, so you want to be a part of that. It’s tough to get hurt now and know that I’m not going to play football for a long time.”

Posluszny didn’t know what to do with himself Monday. There was no point in looking at the tape of the Miami Dolphins, the Jaguars’ next opponent, so he cleaned his locker.

Posluszny is in the fourth season of a six-year, $45 million contract he signed in 2011.

He has never played in a playoff game in his career in Buffalo and Jacksonville, but he has been a steady player.

The Jaguars’ defense may not be the same without him.

REPORT CARD VS. BROWNS

PASSING OFFENSE: D — QB Blake Bortles threw a touchdown pass but he also threw three interceptions. That gives him 10 in 4 1/2 games. The Jaguars love his potential, but he is still struggling with rookie mistakes. He had a 40.3 passer rating.

RUSHING OFFENSE: A — The Jaguars had their best rushing day of the season as RB Denard Robinson, a quarterback in college at Michigan, rushed for 122 yards. He is supposed to be a change-of-pace runner, but he showed he can carry the load, finishing with 22 carries.

PASS DEFENSE: A — The Jaguars handcuffed QB Brian Hoyer, who completed 16 of 41 passes, threw an interception and failed to get his team into the end zone. He had a 46.3 passer rating. He was also sacked three times.

RUSH DEFENSE: A — The Browns came in with the league’s No. 3 rushing offense but gained only 69 yards on 30 carries. RB Ben Tate managed only 36 yards on 16 carries.

SPECIAL TEAMS: B — The Jaguars took advantage when Cleveland PR Jordan Poyer tried to field a punt on his 2-yard line and it bounced off his face mask. Jaguars LB LaRoy Reynolds recovered to set up a touchdown. Ace Sanders had punt returns of 19 and 11 yards.

COACHING: A — Gus Bradley didn’t lose his team when the Jaguars started out 0-6. He stayed positive and kept the team positive, and Jacksonville finally recorded its first victory.