The Giants’ season-long rash of injuries is so contagious it has taken down Leonard Williams.

One of the NFL’s most durable players, Williams suffered a serious elbow injury that likely will keep him out of Sunday’s game against the Cowboys, a source told The Post. The “significant” injury could be season-ending, according to NFL Network.

Williams left Sunday’s loss to the Chargers. He was preparing to return to the game before the plug was pulled.

So, now Williams’ streak of 108 consecutive games played (100 starts) to begin his seven-year career is all but over. And the Giants will be without one of their two legitimate pass-rushers. Williams is tied with rookie Azeez Ojuarli for the team lead with 10 quarterback hits and is second to Ojulari (6.5) with 5.5 sacks, though he hasn’t added to his tally in the last five games.

Williams underwent an MRI in Los Angeles on Sunday and the resulting images went with head athletic trainer Ronnie Barnes and injured quarterback Daniel Jones for review by the Giants-affiliated doctors at the Hospital for Special Surgery, according to coach Joe Judge.

If his season is over, Williams’ 5.5 sacks are the third-highest total of his career. His run defense grade ranks No. 10 among all NFL interior linemen and he was on pace for the second-highest snap count of his career, according to Pro Football Focus.

Williams would be the 10th of the 12 Giants with the highest salary-cap charges in 2021 to miss a game.

For at least one game, the effects of losing Williams could be offset by the Cowboys playing without All-Pro left tackle Tyron Smith, who will miss his fourth game with an ankle injury.

The Giants placed receiver Kadarius Toney on the COVID-19 list after he tested positive Monday. Toney was part of the 10-day, three-city trip to Miami, Tucson, Ariz., and Los Angeles even though he hasn’t played in a game since Nov. 22 due to quad and oblique injuries.

Toney was isolated mid-flight, per source, though protocols require all team personnel to wear masks on the plane at all times.

Toney’s training camp was interrupted by a symptomatic case of COVID-19 and it limited his role during the first two games of the season.

Against the NFL’s second-worst rushing defense, the Giants’ first-half play-calling breakdown was 19 passes to nine runs. Three straight passes — including a second-down screen for a loss of 2 yards by Saquon Barkley — led to a punt after taking over at the Chargers’ 41-yard line with 1:40 to go in the half.

“There was an opportunity to make a big play right there,” Judge said of the screen. “We’ve got to go ahead and execute, make sure we get the blocks started and get Saquon started in space.”

The Giants didn’t even need to gain a first down to attempt a field goal, so running seemed like the way to go. It also would have taken time off the clock and maybe prevented the Chargers’ next score. But the normally conservative Judge was trying to coach aggressively.

“Do we go back and look at it in terms of, ‘Do we run the ball?’ ” Judge said. “We consider that. A lot of times you’ve got to consider the short catch-and-run type of passes as an extension of your running game, as well. We’ve got to make sure we end that with at least three points, but really you want touchdowns in that situation.”