Will 2021 be swan song for Giants’ threesome of Buster Posey, Brandon Belt and Brandon Crawford?

The Giants are in a rebuilding phase, not that you would have noticed by looking at their infield this season.

And you still might not notice by looking at the 2021 infield, which figures to be similar.

Of the five infield positions (including catcher), four are manned by players with large, long-term contracts, so there will be little wiggle room next year barring an unexpected trade.

The Giants have promising young infielders on the way, and highly touted 19-year-old shortstop Marco Luciano is at the top of the list.

2020 review: Shortstop Brandon Crawford and first baseman Brandon Belt finished a lot better than they started. Crawford was hitting .171 on Aug. 9, and Belt opened on the injured list because of a heel issue that required postseason surgery and was hitting .128 on Aug. 15.

Belt wound up enjoying a career year — albeit the year was just a bit more than two months — by hitting .309 with a 1.015 on-base-plus-slugging percentage and Crawford had a .792 OPS, the highest of his career.

Both Brandons subscribed to the innovative theories demonstrated by three new hitting coaches, and it paid off handsomely.

The Giants were a sloppy, error-prone team in the early weeks. With Evan Longoria on the injured list (strained oblique), no one was equipped to handle third base, including Donovan Solano and Wilmer Flores, both better hitters than defenders. Pablo Sandoval, coming off Tommy John surgery, didn’t play third until September.

Order was restored when Crawford and Belt began playing regularly. Solano, who opened in a platoon role, hit his way into the lineup as the second baseman, leading to the decision to move Mauricio Dubón out of the infield and into center.

Flores was best served as the designated hitter and backup to Belt at first. Daniel Robertson arrived in September as a valuable utilityman.

Then there was the catching carousel. Buster Posey opted out of the season after he and his wife adopted twin baby girls who were prematurely born, and that left the Giants with no experienced catchers.

Tyler Heineman and Rob Brantly were on the season-opening roster, and Chadwick Tromp quickly replaced Brantly. Aug. 20 marked the major-league debut of the Giants’ most highly touted prospect, Joey Bart, who experienced growing pains as the No. 1 catcher in the final weeks.

San Francisco Giants’ Joey Bart against the Colorado Rockies during a baseball game in San Francisco, Tuesday, Sept. 22, 2020. (AP Photo/Jeff Chiu)Jeff Chiu / Associated Press

Rising prospects: Of the Giants’ top three prospects (including Bart and outfielder Heliot Ramos), Luciano is the youngest, turning 19 last month, and nobody needed reps in 2020 more. In normal times, he likely would have spent most the summer at Class A Augusta.

Luciano didn’t get his full share at the alternate site in Sacramento (no one did this year), but he’s continuing his training program at the instructional league in Arizona and seems destined to emerge as the Giants’ best homegrown Latin American player since Sandoval.

Others on the rise are 22-year-old Will Wilson — who can play shortstop, second base and third — and third baseman Luis Toribio, 20.

With two catchers of the future — Bart, the second overall pick in 2018, and switch-hitter Patrick Bailey, the 13th overall pick this year — the Giants one day could have an impressive 1-2 punch behind the plate.

Or else they’d need to convert one to a corner infielder or take advantage of the designated-hitter rule, if it still exists in the National League.

San Francisco Giants catcher Buster Posey swings during their game with the Cleveland Indians at Scottsdale Stadium Thursday, March 5, 2020, in Scottsdale, Arizona.Darryl Webb / Darryl Webb

Key question for 2021: Is this the swan song for Posey, Crawford and Belt?

They’re the three remaining players from the championship era, winning a combined seven World Series rings, and all their contracts expire after next season. Posey and Crawford will be 34, Belt 33.

President of baseball operations Farhan Zaidi had no championship connection to Hunter Pence and Sandoval and yet brought both popular players back to the roster, which means he might be open to bringing back at least one of the other three.

The decision will have much to do with how they fare next season. Crawford and Belt showed significant improvements working with the new staff, and Posey was open to the new teachings before he stepped aside.

The decision also will be about the roster makeup at their positions and whether they would like to continue playing in San Francisco in 2022.

San Francisco Giants shortstop Donovan Solano (7) goes hard to his right for Los Angeles Dodgers Los Angeles Dodgers designated hitter Joc Pederson�s infield single during the fourth inning of a baseball game on Thursday, Aug. 27, 2020 in San Francisco, Calif.D. Ross Cameron / Special to The Chronicle

Potential free agents: The Giants have plenty of catching depth, but Zaidi wants more so that Bart could begin next season in the minors and get more seasoning. The aim will be to find a catcher to complement Posey, perhaps one who bats left-handed, a Stephen Vogt/Jason Castro type.

Speaking of which, Vogt and Castro are free agents.

The Giants need someone to play third base when Longoria is off, the role Sandoval was supposed to assume. Preferably a lefty bat. Preferably someone who could play multiple infield positions. Lo and behold, Tommy La Stella is a free agent.

John Shea covers the Giants for The San Francisco Chronicle. Email: jshea@sfchronicle.com Twitter: @JohnSheaHey