SANTA CLARA (AP) — Alex Smith was booed so many times on his home field at Candlestick Park, he has no idea how he might be received by the old fans when he runs out with the Chiefs to face the 49ers on Sunday.

The 2005 No. 1 overall pick, Smith has clearly moved on from his tumultuous tenure with the Niners. Now comfortable and confident in his new job, he’s fresh off last month’s contract extension that could keep him in Kansas City through 2018.

“You learn how fragile this thing is,” Smith said. “This is a ‘what have you done for me lately?’ business and you’ve got to go out every single week and prove it. That’s the deal. I’ve played long enough to know that.”

He certainly hopes the Chiefs (2-2) can build off a commanding 41-14 victory against the Patriots on Monday night that snapped a four-game losing streak at Arrowhead Stadium.

Smith hasn’t spoken to 49ers coach Jim Harbaugh in some time and trades text messages on occasion with Colin Kaepernick, the man who replaced him under center for San Francisco in November 2012.

“There’s also a bunch of new faces and a new stadium that I’ve never been to,” Smith said. “Definitely going against some of those guys I played with for quite a while is kind of funny.”

Tight end Vernon Davis, among so many others still around, long supported Smith.

“I love Alex,” Davis said. “We did a lot of great things together. I’m extremely thankful for the time that he was here. But he’s moved on. He’s doing well. He’s playing good football at the moment. I wish him the best.”

Coach Andy Reid knows a thing or two about potential distractions, having gone back to Philadelphia to face the Eagles in his first season as Chiefs coach last year. Reid isn’t concerned about Smith’s focus when he makes his first appearance at the new Levi’s Stadium.

“He’s been around a while. He knows that you just got to go through the process and play a game,” Reid said. “The emotions, all that stuff, doesn’t necessarily help you.”

Here are some things to watch for Sunday:

Stingy San Francisco ‘D’

The 49ers (2-2) held down one of the league’s best offenses, Nick Foles and the Eagles, without a touchdown and didn’t even allow Philadelphia past midfield until late in Sunday’s 26-21 win. San Francisco didn’t commit a penalty, either.

In his 100th career start, NFC defensive player of the week Antoine Bethea made his first interception since joining San Francisco to go with a forced fumble and seven tackles in last week’s win.

The biggest chore this week, aside from rattling Smith, might be slowing Jamaal Charles after this three-touchdown performance Monday night. And handling the mixing and matching of Charles and fellow RB Knile Davis.

Gore’s gains

Frank Gore produced his best game of the season against the Eagles with a career-long 55-yard touchdown reception and 119 yards on the ground for his first 100-yard rushing game.

Given Gore’s sporadic use in the offense so far, it’s unclear what his role might be Sunday. He had just 35 carries and 139 yards through the first three games.

“They know what I can do,” Gore said. “I’ve just got to continue playing football and when I get the opportunity I’ve got to try my best to make plays. That’s all I’ve got to do.”

Jenkins’ progress

With four catches for 16 yards, Chiefs wideout A.J. Jenkins has been more productive already than during his disappointing stint with San Francisco. The 49ers made him their top draft pick in 2012, but Jenkins didn’t make a single catch while spending most of his rookie year watching from the sidelines.

He was traded to the Chiefs before the ‘13 season.

“A.J.’s done a nice job for us. He does a whole lot of things for us,” Reid said. “A lot of different areas. And so he was a welcomed addition. He goes about his job the right way and, most of all, he has the confidence of the quarterback, who inevitably, has the ball in his hands last. That’s important.”