The career arcs of two quarterbacks passed each other Sunday at FedEx Field like two footballs crossing trajectories. Kirk Cousins was headed upward, his NFL hopes of a lifetime taking flight as he efficiently helped Washington to a 41-10 slaughter of feeble Jacksonville. The other, Robert Griffin III, lost altitude melodramatically for the third straight season. This time, he fell to earth, collapsed on the sideline — lost for weeks, maybe the year — on a play on which he was barely touched.

Griffin, one of the most physically gifted athletes to play the NFL’s most important position, was taken from the field on a cart, his ankle dislocated — bones perhaps broken, though that’s not known yet. In pain he called “excruciating,” Griffin headed to a locker room where doctors, in a procedure he preferred not to remember or describe, put his ankle back “in the right position.”

“I’ll leave the details to the doctors of what happened in that room,” Griffin said.

On his final play, he scrambled, seemed a blink from running out of bounds, then fired back across his body to complete a 19-yard pass that few would attempt and fewer could complete. But like a startled deer that finds itself dodging traffic, his spindly legs and joints seemed to go in all directions. The most asked questions about his injury might have been, “When did it happen? Which step? What caused it? Was he even hit?” And the harsh RGIII answer might be, “It could have been any of them,” because the NFL seldom seems to be in an ecosystem designed for his survival.

“When you look down and something’s not in the right position, you don’t try to get up,” he said.

The Redskins offense shined on Sunday, despite serious injuries to Robert Griffin III and DeSean Jackson. The Washington Post's Gene Wang and Dan Steinberg discuss the impact these injuries will have and how the team will move forward with Kirk Cousins at the helm. (Meghan Sims/The Washington Post)

In a blink, Cousins, a fine but less spectacular athlete and a third-year player with starts that range from 381 yards passing in a loss to the Falcons in Week 15 last season to a quarterback rating of 31.8 in a lopsided loss to the Giants two weeks later, prepared for the chance of a professional lifetime to play — and play and play. Because Griffin will be out a long time, perhaps the rest of this season.

“You always have to be ready,” said Cousins, quick study of the sport, noting his first three thoughts were not about Griffin’s injury but “ ‘Where is my helmet? Where is a football? And where is somebody to throw it to and get warm?’ ” As Coach Jay Gruden said, “It may sound cold, but that’s how the game is.” Next man up.

“If that’s your one shot to show what you’ve got, then you have to be ready to take that shot,” Cousins said. “This is Robert’s team. That job doesn’t change.”

It’s Robert’s team until it isn’t. Wally Pipp had driven in more than 100 runs in back-to-back seasons when he took a day to let Lou Gehrig play. That’s pro sports. It’s your job — no matter how famous you are or what price was paid for you — until somebody else looks better.

Right now, it’s Cousins’s job to create a debate about which quarterback is better. It’s also his task to avoid the opposite conclusion: The backup quarterback looked fine until he got to play — then everybody realized he was just a nice backup. In Cousins’s case, he happens to back up a starter who has difficulty staying on the field.

Some football injuries are awful. As soon as we see them we know the entire trajectory of a career may change. We think, “He may never be the same.” Griffin suffered one of those grotesque, rip at your heart injuries in that playoff game against Seattle in January 2013. It’s still not known how long his knee can remain stable or how good an NFL quarterback he may or may not become.

Other injuries, even those that dislocate a joint or break a bone, are simply part of the sport. They take time to heal. But most players return as the same athlete they were. Their careers are delayed but not fundamentally changed. It’s sad, but it passes.

To give a sense of those different kinds of injury, Griffin came to a postgame news conference on crutches and awkwardly hopped up the steps on one foot. Somebody want to give this guy a hand?

Griffin was demoralized. But not like a running back with a torn anterior cruciate ligament or a pitcher with a ripped labrum. Pros know. There’s “career changing,” and there’s “man, this is going to take a lot of work.”

There’s nothing good about a bad injury. “We’re sick about it,” Gruden said. But there are compensations. As long as Griffin stayed healthy, Cousins was going to sit — this year, maybe longer. He might have left town or been traded without the franchise ever getting a firm grasp of his present ability or potential. He’s probably not Drew Brees. But what if he is?

Now Washington and Gruden will get a full test and a complete answer. Someday, with truth serum, we may find out why Gruden was in such an odd postgame mood, sometimes smiling or quipping, sometimes empathetic with Griffin but often praising Cousins. Maybe he was euphoric at his first victory as an NFL coach, one in which his team had a 449-148 advantage in total yards that screams “brilliant game plan.” Maybe he was deliriously pleased at the sight of 10 sacks by his defense, which gave up the most points in this town in 50 years last season. Maybe he thought, “There’s hope!”

And maybe, perhaps even unconsciously, he thought, “I get to find out about this guy Cousins who looks so good but is never going to get a chance because the previous regime paid a gazillion draft picks for RGIII and the owner, who’s always called the shots here, will back him forever. Now I don’t have to do anything, and my biggest problem will just play itself out without me getting beheaded.”

Just a thought. Because, I promise, that would be my thought.

“I feel like we can win any game with Kirk Cousins,” Gruden said, quickly adding he felt the same about subs in general. “Kirk is a special guy. He started [three] games last year and didn’t have great success, but [he] has a skill set that I feel like is very much suited for what we do. He can handle it mentally. I feel that he can make every throw in the book.”

Asked whether he had any experience with dislocated ankles and the recovery times of others players during his 30 years in football, Gruden said, “You are asking the wrong guy. I got a communications degree.”

Kremlinologists will note he smiled and not for the only time. But 41-10 will do that.

Bad isn’t good. But Washington is going to find out whether Cousins can play. Nobody can block it. If he’s great, he’s great. Have a parade. If he’s good, he’s tradeable, playable or debatable — all positive outcomes. If he’s just okay, then D.C. has a fine backup QB. A bust? He already has shown too much to expect that. On Sunday, he completed 22 of 33 passes for 250 yards, two touchdown, no interceptions and a 109.4 quarterback rating. Griffin’s rating was 109.7 before he left.

Now Cousins will get to play for a 1-1 team that still believes it can have a good season. He gets to exploit the new offensive weapons assembled for Griffin, including DeSean Jackson and Andre Roberts. He gets to show whether he can master Gruden’s offense. And he also will face the pressure, probably severe against good teams, that Griffin would have confronted. Those days under assault have been Cousin’s problem in the past, especially interceptions and fumbles.

It’s likely that someday the main memory of this game will be a Griffin injury that evokes deep sympathy but also calls into question — again — whether he’s durable enough to be a fine NFL quarterback. But it was also the day that ensured Kirk Cousins would get his main chance.

Where is the balance point among all these conflicted emotions? We’re about to find out.

For more by Thomas Boswell, visit washingtonpost.com/boswell.