CARSON, Calif. – It’s amazing how everything can turn on one play.

One week, and one play, the San Francisco 49ers have the look of a playoff team. And one Jimmy Garoppolo blown knee and a week later, nothing came easily. A 29-27 loss to the Los Angeles Chargers looks like the forecast for the remainder of the season for Kyle Shanahan’s bunch.

The 49ers – who still had a chance to win until a C.J. Beathard interception with 2:31 killed all hope – have the look of a squad that will find itself within reach in a lot of games, only to fall short because of its shortcomings at the most important position on the field.

Although their prospects look bleak, no one in the 49ers locker room is ready to concede defeat for the remainder of this season.

That’s evident in the never-say-die mentality with which San Francisco’s players competed throughout Sunday’s contest.

In the days leading up to the game – even after learning last Monday that Garoppolo, their $137.5 million franchise quarterback, would miss the remainder of the season following reconstructive knee surgery – the 49ers remained optimistic about their chances to compete.

Beathard, a second-year pro, was taking over at quarterback. The 2017 third-round pick gave his teammates hope because of the grit that he displayed in five starts last season prior to San Francisco’s trade for and turn to Garoppolo.

“Unfortunately, injuries are part of the game,” wide receiver Pierre Garcon told USA TODAY Sports on Sunday. “If it had to be anybody, we were happy that it was C.J. since he’s had some experience as a starting quarterback and playing with us, and it’s easier now than it was his rookie year. He did a pretty good job today, to tell you the truth.”

Beathard completed 23 of 37 passes for 298 yards, two touchdowns and two interceptions in the loss to the Chargers, and he showed why Garcon and his teammates have confidence in him. He also demonstrated why 49ers brass felt more comfortable moving forward with him for the year instead of plugging in a free agent. Beathard at least knows the system and has continued to grow in his understanding of his coaches’ philosophies. He will provide greater continuity, the 49ers believe, for the time being.

But Beathard also showed why the 49ers saw the need last year to trade for Garoppolo and build the team around him for the long term.

On Sunday, Beathard and the 49ers did get off to a promising start. Coach and play-caller Kyle Shanahan did a good job of utilizing the run game and a number of misdirection plays to help ease pressure on the passing attack. Beathard looked comfortable, particularly on the six-play, 56-yard touchdown drive capped by a 2-yard play-action pass to Kendrick Bourne in back of the end zone to give his team a 14-0 lead midway through the first quarter.

But as the game progressed, and as the Chargers started to get a read on Beathard’s tendencies, the young signal caller was stymied. He had three batted passes and was hindered by increased pressure from Los Angeles' pass rush.

Shanahan described Beathard as a gamer this week. On Sunday, he never wilted, even when both of his starting tackles and two wide receivers got hurt. He continued to stand in the pocket and deliver passes despite taking hard hits (he drew three roughing-the passer penalties – and these looked legit, unlike some we’ve seen around the league this season – and threw completions on all three). He slipped defenders here and there and rushed for key yards. On one such play late in the game, Beathard gave the 49ers a scare as he required medical attention after getting hit hard on a run. But it turned out Beathard only got the wind knocked out of him, and he returned to the game.

“He’s a tough (expletive) — he keeps getting licked and he keeps fighting,” right tackle Mike McGlinchey said. “That’s who he is. We all know it and now everybody else knows it. We love going to battle with a guy like him.”

But the problem with Beathard for now is that those difference-maker throws that Philip Rivers gave the Chargers here and there, and that Garoppolo likely would have given San Francisco, were hard to come by at various points in the second half once the 49ers found themselves trailing and forced to throw more.

So too was the recognition of a savvy veteran quarterback. On the last offensive 49ers play of the game – with his team trailing 29-27 with 2:31 left – Beathard underestimated safety Derwin James, who blitzed off the edge untouched. Beathard said he saw the rookie defensive back but thought he could find a downfield receiver in time. He did not, and James jarred the ball free, leaving defensive lineman Isaac Rochell to catch it for an interception that sealed the win.

Shanahan said that the right play would have been to quickly check down to an underneath receiver.

“The last play was unfortunate. I wish he had saw that blitz coming and got rid of it,” Shanahan said before elaborating on the proper response for Beathard.

These are the lessons that Beathard must learn, and those are the kinds of shortcomings that will hold the 49ers in check for much – if not all – of this season.

The margin for error for every other 49ers player is so small with Garoppolo now gone. The same goes for Shanahan. He can’t rely on the instincts of a crafty veteran to help negate play breakdowns. Shanahan did a great job with his play design to give Beathard open targets. But, there were plenty of times where Beathard couldn't create on his own when he didn't have an obvious go-to receiver.

A pick-six on the second play of the game certainly helped gave San Francisco a quick 7-0 lead. But, once the 49ers fell behind, any little penalty or missed tackle seemed like massive obstacles to overcome.

Give the 49ers credit, though, as they continued to compete after instances where it looked like they certainly would crumble.

The third quarter featured a potentially demoralizing play for the 49ers. Beathard and found a rhythm going to Garcon twice, then Bourne and then 44 and then a pass that appeared a little wide went off of tight end Garrett Celek’s hands and into those of CB Trevor Williams, who returned it all the way to the chargers 12. The 49ers wound up forcing the Chargers to settle for a field goal to extend their lead to 26-17. But Beathard came back with an 82-yard touchdown pass to tight end George Kittle, and the defense denied the Chargers on a key third down and San Francisco wound up getting a field goal (27-26) out of that.

Late in the fourth quarter, Melvin Gordon and the Chargers looked poised to grind out the clock with a series of hard-to-stop runs. But San Francisco’s defenders regrouped, contained the L.A. offense and got the ball back for the offense with just less than four minutes left. Then came the dagger: the interception to Rochell. There were, however, missed tackles earlier in the game both on defense and special teams that could have led to a different outcome.

The 49ers' entire roster must play better and then some to have a chance. So, too, must Beathard. But at 1-3 and without its catalyst, San Francisco is facing a bleak outlook.

Follow Mike Jones on Twitter @ByMikeJones.