PHILADELPHIA (CBS) — A huge piece of encouragement greeted the Eagles on their first day of training camp: Pro Bowl right guard Brandon Brooks was back. Brooks suffered a torn Achilles in the Eagles’ season-ending playoff loss to the New Orleans Saints in January, and once Brooks went, the Eagles went.

Brooks is just one of several critical pieces the Eagles will need to go right this season.

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As a whole, the offensive line appears strong again. But it has to be noted that Brooks, Lane Johnson, Jason Peters and Jason Kelce are all coming off injuries.

Johnson fell off somewhat last year, mostly due to a sprained MCL in his left knee that plagued him during the second half of the season.

Above all, Johnson could be biggest X factor on offense. If he goes down, the Eagles’ offensive line may go with him.

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This season, he’ll go against the Cowboys’ Demarcus Lawrence twice and he’ll see the Chicago Bears’ Khalil Mack — two of the NFL’s top defensive ends who Johnson shut out last year.

Washington’s Jonathan Allen could be a problem, too, after leading the Redskins’ defensive linemen in tackles (61), along with eight sacks.

In October, the Eagles travel to Minnesota, where Danielle Hunter is trouble. Last year, the 24-year-old collected a career-high 72 tackles and 14.5 sacks.

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Against the Eagles and Johnson, Hunter had a sack, two solo tackles and two quarterback hits last year in the Vikings’ 23-21 win at Lincoln Financial Field.

Johnson had his problems in that game, getting beaten badly by Stephen Weatherly on an inside rush that knocked the ball loose from Carson Wentz and right to Linval Joseph, who huffed-and-puffed 64 yards for a touchdown and a 10-3 Vikings’ lead with 4:12 left in the half.

Defensively, the X factor is Pro Bowl safety Malcolm Jenkins.

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Like Johnson, Jenkins plays a heart-and-soul role on the Eagles’ defense. In the last two years, Jenkins has played linebacker, nickel slot, safety and cornerback.

He didn’t miss a defensive play in 2018, playing 1,177 snaps. With safety Rodney McLeod coming off ACL/MCL injuries, the Eagles have Andrew Sendejo and Blake Countess as their safeties. That could be trouble if the Eagles start the season with them.

Jenkins has been a pro in dealing with his contract situation, which has gone silent, because of him. Eagles’ executive vice-president of football operations Howie Roseman knows and realizes Jenkins’ value on and off the field.

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Jenkins is invaluable. He may not be able to cover like he used to two years ago, but he knows Jim Schwartz’s defense. A happy, and healthy Jenkins is a factor that the Eagles could ill afford to lose in 2019.