Shortly before the Washington Redskins took the field Friday afternoon for their second practice session of training camp, they learned starting left defensive end Jarvis Jenkins would be suspended for the first four games of the season because he had tested positive for a banned substance.

For the third time in the first 48 hours of training camp, the Redskins’ defensive depth was put to the test. News of Jenkins’s suspension came one day after Adam Carriker, who would have competed with Jenkins for playing time at left defensive end, underwent a third surgery on a problematic right quadriceps tendon — a procedure that will require at least a four- to five-month recovery.

Then late Thursday night the team learned second-year inside linebacker Keenan Robinson — drafted last year to eventually take over for 38-year-old London Fletcher — had torn his left pectoral muscle and will need roughly three to five months to heal following surgery.

Jenkins’s absence hurts the most and is the most exasperating because it could have been avoided. Jenkins spoke to reporters after Friday’s practice and said the banned substance was an ingredient in a pre-workout and recovery supplement. The substance is often taken by women with breast cancer but also is used as a masking agent for steroids, he said.

The NFL and NFL Players Association have an arrangement with the company NSF International to certify supplements that don’t contain any substances banned by the sport.

The Post’s Jason Reid, Dan Steinberg and Jonathan Forsythe discuss what story lines out of Redskins training camp they will be following outside of Robert Griffin III’s recovery. (Post Sports Live)

Jenkins, who is entering his third season as a pro, said he got the supplement from GNC but acknowledged that it didn’t bear the label that players are told to look for.

“It was an honest mistake, but the NFL has rules,” Jenkins said. “I was trying to do everything by the rules, and obviously they tell us to take supplements that have ‘NSF’ on it. That’s tested by the NFL. I made my choice, and I take recovery and pre-workout and supplements, and nobody knows what’s in those supplements, even though the label doesn’t say it’s a banned substance. But they tell us that we’re responsible for what’s in our body, and I’m being led as an example. Again, I apologize to the Redskins. They tell me to do everything by the book. It was a simple mistake, but I’m going to accept my responsibilities like a man, and with the suspension, obviously, I’m letting my team down and the defense, but I’m going to come back stronger from this.”

Jenkins said he learned of the positive drug test in March and appealed the ruling. He learned Friday he had lost his appeal. This is the eighth drug-related suspension the Redskins have had in the past three years.

“Any time a person has a suspension, it’s always a tough blow to your football team,” Redskins Coach Mike Shanahan said. “Hopefully, our players will learn that without the label of ‘NSF,’ you can’t take any supplements, because you never know what’s going to be in those supplements. So we’ll pay the price for it. He’ll be suspended for four games, and hopefully our players will learn from it.”

Shanahan later added, “Something like Keenan, it was a freak accident. I saw it happen early: It was no contact. He put his arm back, and you just feel sorry for him because he’s worked so hard to get back and be in great shape.”

Now the Redskins must come up with contingency plans to compensate for the loss of a starter and two backups who would have been used in their defensive end and linebacker rotations as well as on special teams.

Eighth-year veteran Kedric Golston and second-year pro Chris Baker will get the first crack at filling the voids left by Jenkins’s suspension and Carriker’s surgery. The team re-signed both players in the offseason because officials were aware of the uncertainty regarding Carriker’s health.

Golston served as a backup swing end the last two seasons, and in his last opportunity as a starter (2010), he started 13 games, recording 35 tackles and one pass breakup.

Baker, meantime, spent 2011 on Washington’s practice squad and last season earned a spot on the 53-man roster as a backup nose tackle. This offseason, he has worked almost exclusively as a left end behind Jenkins.

Now those two will vie for the interim starting job. Whichever player doesn’t start still figures to see a fair amount of action; defensive coordinator Jim Haslett rotates linemen often to keep his players fresh.

Meanwhile, at inside linebacker, fourth-year pro Roddrick Muckelroy appears poised to take over as the backup to Fletcher, who is responsible for making all the defensive calls, essentially serving as the quarterback of his unit. Signed in November after Robinson’s first injury, the Texas product didn’t see time in a game for Washington last season. But he played in a combined 19 games for Cincinnati in 2010 and 2012. Muckelroy missed the 2011 season because of injury.

“I gained a lot of confidence working with the [starters],” Muckelroy said. “Coach threw the playbook at me, and I was able to respond well. I’m just trying to take advantage of that here in training camp and taking it from the class to the field.”