The Packers won’t know the severity of defensive end Mike Neal’s knee injury until probably Wednesday, but if he’s lost for any extended period of time, it would be a significant blow to the defense.

Neal, who got his feet tangled up in a non-contact drill early in Tuesday’s practice and went down clutching his left knee, was projected when training camp began as the replacement for departed free agent Cullen Jenkins on the defensive line.

“The way Mike fell; I saw it from the special teams drill, just the reaction of everybody. It makes you nervous,” Head Coach Mike McCarthy said.

Nervous because Neal and his powerful 6-3, 294-pound frame give the defense the closest thing it has to another Jenkins, who was an every-down player and multi-tasker in Defensive Coordinator Dom Capers’ 3-4 scheme – stout against the run and able to provide interior pass-rush. Neal, a second-round draft pick in 2010 out of Purdue, had been working with the first team defense since being medically cleared the first week of camp following a season-ending shoulder injury last October.

Tuesday afternoon, Neal re-tweeted former teammate Daryn Colledge, “Thanks!! Im A OK!”

The Packers can only hope they get good news after Neal is evaluated by team doctors and an MRI, if necessary, is taken on the knee. Neal left the practice field under his own power on Tuesday, walking with a limp.

Following practice, Neal’s defensive linemates said they hadn’t talked to him and hadn’t heard anything. They’re simply hoping for the best.

The immediate burden will fall on second-year pro C.J. Wilson and third-year man Jarius Wynn to pick up the slack.

Wilson, a seventh-round draft pick last year, and Wynn, a sixth-rounder in 2009, came through when injuries hit the defensive line last year, and by all accounts they continue to improve.

Last season, Neal was lost for the year after Week 5, Jenkins missed five games and Ryan Pickett missed two contests. Thought by many to be too young for the duty, Wilson and Wynn held up fine, with help from veteran free-agent pickup Howard Green. The two didn’t make a lot of standout plays, but they did combine for 3½ sacks, including the postseason.

Wilson is probably the better of the two against the run, while Wynn is the more natural pass-rusher. Both players took snaps with the first-team defense on Tuesday after Neal’s injury.

Capers has said he thought Wilson was one of the most improved players on the defense down the stretch in 2010, and he appears to have bulked up a bit in the offseason.

“They’re both coming into their own,” Pickett said. “They’re not young players anymore. A lot is expected out of them right now and I think they’re both doing a great job answering the bell.”

Nose tackle B.J. Raji echoed those sentiments, expressing confidence Wilson and Wynn are up to the task.

“I’m not a coach, but they’re definitely getting better as far as the technique,” Raji said. “We’ll know more after these next couple (preseason) games with the play time being a little different, but so far I think they’re doing a great job.”

Their jobs may have just fallen under an elevated level of scrutiny, depending on the news coming on Wednesday.

“You have some young players there that have to be ready to go, sooner than later,” McCarthy said.

Neal’s injury wasn’t the only tense moment in practice on Tuesday.

During a seven-on-seven drill late in practice, safety Anthony Levine and cornerback Pat Lee collided as receivers Randall Cobb and Jermichael Finley crossed paths. Lee got up and was fine, but Levine stayed down and was attended to for several minutes. He eventually got up and walked off the field, slowly and aided by medical personnel.

Levine, who is making a run at the No. 4 safety spot behind Nick Collins, Morgan Burnett and Charlie Peprah, was still being evaluated after practice, McCarthy said. Jarrett Bush replaced Levine at safety during the final period of practice and intercepted a pass to end the two-minute drill.

In other injury news, tight end Spencer Havner sat out practice with a hamstring injury, while offensive lineman Marshall Newhouse sprained a knee but returned and finished the workout.

Tight ends Andrew Quarless (groin) and Tom Crabtree (ribs) returned to practice after sitting out on Monday.

Also returning was running back Alex Green, the third-round draft pick, who missed the preseason opener in Cleveland with a quadriceps injury.

Green took snaps with both the regular offense and the scout-team unit at running back, but he did not resume his kickoff-return duties. Cobb took the first kick-return rep on Tuesday.

Possessing good hands and coming from a spread offense at Hawaii, Green is a candidate to play the role of third-down back as a rookie, but without a regular offseason he needs all the work picking up blitzes that he can get.

“Missing any reps hurts, and missing the blitzes, the pickups, it hurt me a little bit,” Green said. “Being out there is a lot better than watching film. Getting reps is key, and that’s my goal this whole week.”