SANTA CLARA, Calif. -- San Francisco 49ers coach Chip Kelly made just about the only move he could in hopes of jump-starting his sagging offense when he announced Tuesday that Colin Kaepernick is replacing Blaine Gabbert as starting quarterback.

For an offense that sits 31st in the NFL in yards per game and 32nd in yards per play, it's the right move at the right time. But it also probably isn't going to be some sort of panacea that turns the Niners' season around.

Sure, Kaepernick is the most accomplished of San Francisco's quarterbacks, once leading the 49ers to within a completed fade pattern of a Super Bowl title. But this version of Kaepernick isn't the same as that one. Kelly has repeatedly made that clear.

"He’s not the same guy he was in 2013; just look at him physically," Kelly said. "That’s the point I tried to make when people brought that up."

If he were, Kaepernick already would have been the starter. Over the past six weeks, Kelly said the biggest difference he has seen in Kaepernick is his physical condition. Kaepernick has gotten bigger and stronger, even if he's not back to the 225-pound playing weight when he was at his best.

At 1-4, the Niners clearly weren't going anywhere with Gabbert as the starter. And Kaepernick deserves an opportunity to see if he can deliver on the idea that he's a good fit for Kelly's offense.

But not only is Kaepernick not the same guy physically he was at his best, he also doesn't have the dominant defense and talented skill-position players surrounding him now like he did then. Which puts the onus on Kaepernick to do things that he hasn't proved capable of on a consistent basis in his career.

Back when Kelly first discussed what he was looking for in his starting quarterback, he ran down a long list of things he wants, but at the top of the list were accuracy and decision-making. Gabbert didn't show much of a knack for either. When last we saw Kaepernick, he didn't either.

When Kaepernick was the starter in the first half of last season, he ranked 27th in completion percentage, 29th in yards per attempt and 26th in QBR. Like Gabbert, he also struggled as a pocket passer, something that is unlikely to be improved given his long layoff. Throwing from inside the pocket last season, Kaepernick had the second-worst QBR of all quarterbacks (rating only above Gabbert) and was sacked on 10 percent of his dropbacks, the highest rate in the NFL.

Kaepernick did once offer a lot of promise when he had a 78.8 QBR while in the pocket in 2012, but again, he doesn't have players like Frank Gore, Michael Crabtree, Vernon Davis and one of the best offensive lines in the league to help his cause now. Kaepernick also has never completed more than 62.4 percent of his throws in a season.

Kelly's offense asks for quick, smart decisions with accurate passes and he made it clear Tuesday that it won't be any different with Kaepernick behind center.

"It doesn’t change," Kelly said. "They’re both similar type of athletes in terms of what they can do. They both run, they both throw, so I don’t think you’ll see schematically we’re not going to change our entire offense to do this. You’re not dealing with two different types of athletic people. Blaine had 70 yards rushing in our last game on 10 attempts so Blaine can run and do what we’re doing offensively. Kap can do what we’re doing."

Well, maybe not all of it but he can do at least some of it, namely the things Gabbert was already doing.

If there's any place where Kaepernick might be able to make a difference, it might be in the running game. While Gabbert had an inflated QBR because he's leading the NFL in rushing yards by a quarterback, he doesn't have the same running ability as Kaepernick. Kelly's preferred zone-read rushing attack could play into Kaepernick's hands.

The 49ers have run zone-read plays 99 times this season, more than three times as much as the next closest team. For his career, Kaepernick is averaging 4.54 yards per rush on zone-reads, while opting to keep the ball 32 percent of the time. It's possible Kaepernick will be able to turn some of the short or medium gains for Gabbert into bigger plays.

Kaepernick also could be better throwing the deep ball. Gabbert is 3-of-14 this season on passes thrown 20 yards or more downfield, second worst in the NFL. Kaepernick has completed 38.1 percent of such throws in his career with 16 touchdowns and 11 interceptions.

Despite all of that, the reality for the 49ers remains the same. Even if Kaepernick can offer slight improvement in a couple of areas, it's hard to see him returning to the form that made him one of the league's brightest ascending stars.

"He physically is not where he was pre-injury," Kelly said. "Before that, he was a little bit bigger, a little bit faster and a little bit stronger. He will admit that. He’s worked as hard as anybody I’ve ever been around in terms of his rehab."

Now, Kaepernick will get his chance to prove he can help turn the Niners' season around. If he does, it would go a long way in doing the same for his career. But without the help of a great defense and supporting cast on offense, chances are the results won't be much different from what Gabbert brought to the table.

And the ultimate conclusion could be: For the Niners to become the type of offense Kelly hopes to be, the offense will need a quarterback who isn't in the building right now.