SANTA CLARA, Calif. — The San Francisco 49ers went from the second-best team in the NFL to the second-best in the Bay Area in a matter of 10 months.

Three chief reasons for the collapse are cited: Distractions, injuries and quarterback Colin Kaepernick.

Before the highly anticipated season had as much as kicked off, the 49ers reportedly were shopping their coach, their star outside linebacker had been suspended for nine games and one of their starting defensive ends was being accused of domestic violence.

The talent-laden 49ers held things together … for all of seven quarters. They won the opener at Dallas and held a 20-7 lead over Chicago in the grand opening of Levi’s Stadium before the weight of the distractions took their toll.

The 49ers went on to lose to the Bears, then lost seven other times — including to a previously one-win Oakland team — over the next 14 weeks, formally denying themselves a shot at the title without ever taking a postseason snap.

The 49ers got Aldon Smith back from his suspension in the 10th week of the season. That pretty much exhausts the good news personnel-wise.

Fellow All-Pro linebacker NaVorro Bowman never returned from a knee injury sustained last season.

On defense, by season’s end the 49ers were playing without four of their top six players in the secondary, three of their top five linebackers, their top two nose tackles and the aforementioned troubled defensive end, Ray McDonald, whose contract was terminated after a second accusation.

Despite it all, defense wasn’t the team’s problem. The 49ers’ rushing total went down and sack figure went up, both due in no small part to going without their starting center and right tackle for most of the season.

This was the year coach Jim Harbaugh was supposed to take Kaepernick to greatness. Instead, Kaepernick drove Harbaugh to Michigan.

One of the league’s best running threat among quarterbacks stopped running (26 or fewer yards in nine of the team’s first 13 games), stopped throwing to tight end Vernon Davis (26 catches for 245 yards and two touchdowns one season after going 52-850-13) and pretty much stopped playing after three quarters (not a single fourth-quarter touchdown pass all season).

And as if to prevent the formal documentation of said statistics, Kaepernick stopped doing one other thing. He stopped answering the media’s questions.

The 49ers went from a team that complemented a great defense with a steady offense, to a club whose offense rarely was worthy of being complimented at all.

In the end, management decided a future without Harbaugh was the best way to make a U-turn. Certainly, there are two reasons for optimism.

Chances are the 49ers won’t try to trade its new coach next summer and McDonald won’t be around, so there figures to be fewer distractions.

And even if one-third of the starting 22 take headers while surfing on early winter vacation, it’s unlikely the injury toll will be as damaging in 2015.

So will the 49ers immediately be capable of challenging the Seattle Seahawks again? That could very well come down to Problem No. 3 — Kaepernick — something club management acknowledged at its going-away press conference for Harbaugh.

“The quarterback position is the most important position in all of sports. Period,” 49ers CEO Jed York insisted. “I think Colin did some things very well this year. And there are things he needs to improve on. And that’s something that is very important.”