The 2008 fifth-round pick of the Tampa Bay Buccaneers hadn’t played in an NFL game since 2013 and hadn’t thrown a pass in the league since Dec. 11, 2011, but Washington was desperate after losing quarterbacks Alex Smith and Colt McCoy to broken legs. Johnson was signed Wednesday to be the team’s emergency backup after Mark Sanchez, who was signed Nov. 19, was named the starter.

The emergency in Sunday’s ­40-16 loss to the New York Giants turned out not to be an injury to Sanchez but rather a horrid first half for the Redskins’ offense, so Johnson made his ­Redskins ­debut at 5:31 of the third quarter. Washington had fallen behind 40-0 before Johnson entered, and he led a pair of touchdown drives coupled with two-point conversions.

Coach Jay Gruden said Johnson would start Washington’s game Sunday at the Jacksonville Jaguars.

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“Honestly, [this is] something I’ve been doing for the past six years,” Johnson said. “I’ve been cut so much, been picked up week of games. Got picked up one time the day before a game. The poise was there within myself because I just had to remember what I did before. What is there to lose? What is there really to lose when you come from getting cut so much, not having an opportunity, wanting an opportunity? This is your opportunity, so you take it.”

A lot has happened to Johnson in the past 13 days. He was the No. 1 pick in the Alliance of American Football draft Nov. 27 before arriving in Washington on Tuesday. He didn’t take a snap with the first team in practice during the week; the Redskins’ focus was on preparing Sanchez. Johnson played the Madden NFL video game in the evenings in an attempt to learn his new teammates’ names.

Gruden had planned to start Sanchez for the rest of the season, but the veteran had one of the worst outings of his career. He completed 6 of 14 passes for 38 yards with two interceptions, including one returned for a touchdown. He was also sacked five times, and his passer rating of 10.7 was nearly 30 points worse than what a quarterback earns for throwing an incompletion on every attempt. The only time Sanchez had recorded a worse rating was as a New York Jets rookie in a 16-13 loss to the Buffalo Bills on Oct. 18, 2009.

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By the time the fourth quarter rolled around, Sanchez was sitting on the bench in an oversized coat with the hood pulled up, surrounded by enormous swaths of empty seats at FedEx Field.

“It was tough sledding, for sure,” Sanchez said. “That’s easy to figure out. We were trying to establish the run game hard in the beginning, and we just kind of ran into a little bit of a buzz saw there and got ourselves into some tough situations.”

Johnson injected some energy into the offense after it had just 51 first-half yards and went 0 for 7 on third down. The Redskins managed just two first downs in the first half and had 30 passing yards. The 34-0 halftime score was the team’s largest home deficit at that point of the game since at least 1940, according to Pro Football Reference.

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The Redskins actually began to move the ball after Johnson entered, although it came against the Giants’ reserves. Johnson threw for 195 yards with a touchdown and an interception, and he led the Redskins with 45 rushing yards and a touchdown on seven carries. His mobility proved to be a huge asset behind the team’s injury-riddled offensive line.

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“I thought, at the end of the day, if we are having trouble protecting, I need somebody who can move around and make some plays,” Gruden said. “The ability to scramble and make some plays with his legs, mix in some [run-pass options] and zone reads — all that stuff.”