Chargers to work out Robert Griffin III

Since Philip Rivers became the Chargers’ starting quarterback, whoever has served as his backup has had one of the NFL’s most predictable jobs.

You stand, on the sideline, probably in an officially licensed ball cap, maybe signaling in plays, maybe holding the ball on extra points and field goals. But throwing passes? You probably won’t be doing much of that.

Rivers hasn’t missed a start in over a decade — a rare example of durability in a league where muscles tear, bones break and ligaments stretch.

Still, having a “Plan B” is still required, and Tuesday, the Chargers might be tinkering with theirs.

According to a league source, former Heisman Trophy winner Robert Griffin III will work out for the Chargers with training camp set to begin in less than a week.

Currently, the team has veteran Kellen Clemens and undrafted free agents Mike Bercovici and Eli Jenkins on the roster.

Since taking over as the starter on Sept. 11, 2006, Rivers has been under center for 176 straight games. During that time, Rivers has been backed up by only three players — Billy Volek, Charlie Whitehurst and Clemens.

The team re-signed Clemens this offseason. In three seasons with the Chargers, Clemens has thrown only 10 passes during the regular season.

Griffin’s NFL career has been on a downward trajectory since he electrified the league as a rookie in Washington after starring at Baylor. In 15 starts as a rookie, Griffin threw for 3,200 yards and ran for 815 more, leading his team to the postseason and earning a Pro Bowl spot.

But in the playoffs, Griffin tore multiple ligaments in his right knee as he tried to recover a low snap.

Griffin’s play suffered in his second season with Washington as his relationships with the team’s coaches deteriorated as his production suffered.

He was released in 2016 and Griffin signed with Cleveland, where he became the Browns’ starter. However in the first game of the season, Griffin fractured a bone in his left shoulder.

He returned in December for the Browns’ final four games, including Cleveland’s 20-17 win against the Chargers. In that game, Griffin completed 17 of 25 passes for 164 yards while running six times for 42 yards.

His 2016 season ended with 232 yards passing, his most since 2014, in a three-point loss to the Pittsburgh Steelers. Cleveland released him in March after acquiring quarterback Brock Osweiler in a trade with Houston.

The Chargers will be Griffin’s first visit as a free agent after he spent most of the summer working out in Florida with Pep Hamilton, the former offensive coordinator for the Indianapolis Colts who coached Griffin in Cleveland last season.

Chargers coach Anthony Lynn has expressed a desire to eventually get younger in the quarterback meeting room. Rivers will turn 36 in December and Clemens is 34.

Griffin turned 27 in February.

dan.woike@latimes.com

Twitter: @DanWoikeSports