In his conference call about the cut-down day 53-man roster, 49ers coach Chip Kelly made a joke that cracked up the reporters on the line. His interrogater noted that Colin Kaepernick had improved dramatically from his first preseason game to his second, and asked if Chip expected Kap to get better and better as the season went on?

The coach answered:

That's my expectation for every player on this team. I would hope our guys don't go backwards. We'd be doing a real bad job of coaching if that was the case.

Behind this mild jest is an important subtlety. The ability of players is constantly changing, a race against the clock between accumulated injuries and aging, versus the irreplaceable experience that veterans accumulate.

Fans sometimes fix an image of a player and lock it in: “he sucks,” “no separation,” “happy feet.” Sometimes that’s even true.

But more often, player’s ability is changing game to game and series to series. Success builds confidence, and experience deepens knowledge and instinct. But getting humiliated in the full spotlight of national TV coverage can break a player’s spirit or embolden foes, too. Players do get worse sometimes.

Especially with young players, a coach needs to project how they will develop through the year, and make an educated guess whether they have the chops to even survive long enough to get better.

Sitting on the bench for a year or two is a big help in digesting all of that, if a player gets that luxury. And most players don’t hit their physical peak until 25 or 26, so they have some time to adjust. But they’re often competing against wise and crafty veterans for those last roster spots.

With older veterans, there is less flux. It’s no surprise that older quarterbacks dominate the league, given the intense mental demands of the position. Even with the special rules protecting QBs from injury, it requires years to convert tape watching and scheme diagrams into the muscle memory needed to diagnose and adjust to coverages on the fly. But you need luck to stay healthy long enough to get there.

There is no substitute for regular season experience — not practice, not preseason games, not film study and drills. But every second on a field, real game or not, brings the chance of injuries that can stymie your progress — or end your career in a second. That’s the razor’s edge of player development.

Coaches with lots of college experience, like Chip Kelly, have an advantage in sensing player’s development. They start with 18 year olds who often have never lived on their own, done laundry or had a serious relationship, and thrust those players into the national spotlight. But, as expected, Chip has had to adjust to dealing with adults who bristle against strict rules and the loco parentis moral authority of a college coach.

All of this makes constructing a roster incredibly complicated. You can’t just pick whoever is the best player at every position right now. You need a mix of younger players (bigger risk but healthier, with potential for future stardom) and veterans with savvy, leadership and reliability (but often lower ceilings, and always higher salaries).

It’s a series of gambles, and the art is finding offsetting risks that hedge each other.

Quarterback is especially difficult. Kaepernick had early (and perhaps unrealistic) success because his skill set fit the then-new zone read offense so well. But once NFL defenses adjusted, his underdeveloped skills in reading defenses and nevigating the pocket caught up with him.

I’m partial to Green Bay’s slow growth approach for QBs, and it’s hard to argue with their success. I worry about Carson Wentz in Philadelphia, for example. The Eagles hit the jackpot getting a first and a fourth for Sam Bradford, but that doesn’t mean they have to start Carson Wentz next week.

Starting right away will expose him to further injury — he already has two broken ribs in just one half of preseason football — because he doesn't have a sense yet for the speed of NFL pass rushers (and his offensive line is questionable).

It could also just ruin his confidence because of the huge leap from North Dakota State to the pros. Or both. He’s a huge asset for the Eagles, and I don’t think they’re protecting him properly if (as reported) they plan to start him right away.

Can Kaepernick get back on track, and resume the growth pattern that Chip Kelly expects from all of his players? No one knows yet. But my sense is that Chip has a good understanding of the difficulties involved and how to tackle them.

Playing behind Gabbert (at least through the brutal first four games of this season) is a smart approach, and his dramatic improvement just from his first to second preseason game is a good sign that the answer is yes.