Chargers’ Jason Verrett out for season with torn Achilles tendon

Training camp hasn’t officially begun and the Chargers have lost a key player to a season-ending injury.

Cornerback Jason Verrett, who has been plagued with knee injuries the past two seasons, suffered a torn Achilles tendon during conditioning drills Friday, a day before the team was set to open training camp in Costa Mesa.

Verrett, a former first-round pick and Pro Bowler, has had two knee operations that forced him to miss almost all of last season and most of the 2016 campaign. He participated in the team’s offseason program this summer and was set to bolster an already strong secondary in 2018.

“He was healthy in the offseason program and we were probably more cautious than we had to be. We probably held him back, and he probably wanted to do a lot more. We tried to gently ramp him up, just on volume. But he’s a competitive player,” Chargers general manager Tom Telesco said of Verrett on Thursday. “He’s full go. He wants to be full go. If you’re going to play football, you’ve got to play full go.”

Four games into the 2016 season, Verrett tore his ACL, costing him the season. Then after he returned partway through training camp last year, the Chargers shut Verrett down after the team’s opener and he underwent another surgery on his left knee.

He’s played in just 25 games since the Chargers selected him with the No. 25 pick in the 2014 NFL Draft. Quarterback Johnny Manziel is the only player selected in the 2014 first round to play in fewer games than Verrett.

Verrett’s in the final year of his original rookie contract and will be a free agent next season. He joins Hunter Henry as the second Chargers player to be lost for the 2018-19 season because of injury.

Without Verrett last season, the Chargers defense still flourished. His replacement opposite Casey Hayward, Trevor Williams, had a very strong season and rookie Desmond King played well as the team’s slot cornerback.

Still, the team had been excited about adding Verrett — excitement that’s now turned to disbelief.

More injuries

Chargers first-round pick Derwin James will begin training camp on the sidelines with a muscle injury suffered between the end of the team’s offseason program and the start of the preseason.

The injury isn’t considered serious and James is day to day.

In addition to James spot on the Non-Football Injury list (NFI), the team also put Verrett, Henry, and second-year guard Forrest Lamp on the Physically Unable to Perform (PUP) list. Reserve offensive lineman Brett Boyko will also begin the year on the NFI list.

Lamp, the Chargers’ second-round pick a year ago, missed all of last season after suffering a season-ending knee injury in training camp. He’s still recovering from a second operation on that knee this spring, and Telesco said he’s unsure when he’ll be back.

“The knee is coming along pretty well, but he needs to get back into football condition as well. We’ll see how that goes,” Telesco said Thursday. “We don’t know what he’ll have this camp — it remains to be seen. But yeah, he needs snaps. There’s no doubt about it.”

Players cannot practice while on the PUP or NFI lists, though they can be reinstated at any time during the preseason.

dan.woike@latimes.com

Twitter: @DanWoikeSports