BOSTON – The retired numbers on display at Fenway Park invoke memories of Pedro Martinez (45), Carl Yastrzemski (8), Ted Williams (9) and other Boston Red Sox greats.

Williams may have been the best of them all. But he was not the most valuable player ever to wear a Boston uniform.

That man will play his final regular season game Sunday afternoon at Fenway.

No one has meant more to the Red Sox than David Ortiz.

Before Ortiz came to Boston in 2003, Red Sox fans were enduring taunts of “1918,” the last year the franchise had won a World Series. Since then the team has won three World Series titles – in 2004, 2007 and 2013. Without Ortiz’s postseason heroics, Boston does not win at least two of those championships. Maybe all three.

At age 40, he is retiring from the sport despite remaining one of the top hitters in the major leagues. He has been a critical factor in Boston’s hunt for another title this fall.

“It’s not like he’s going out (with a whimper),” said Baltimore manager Buck Showalter. “He’s a very effective player. I hope he doesn’t change his mind.”

Ortiz is a candidate for the American League MVP Award this season, batting .316 with 38 home runs and 127 RBI. He leads the league in doubles (48) and slugging percentage (.625).

“I even find myself in awe of what he’s doing at times,” Boston manager John Farrell said. “His pulse never changes in those key moments and that’s why I think he’s been able to be such a great performer in key spots. We’re going to miss him.”

Ortiz performs in the clutch. And his clubhouse presence combines individual charisma with a will to win, leading by both his words and example.

“He’s been a great player for a long, long time,” said New York Yankees manager Joe Girardi. “And he has been really good for the game of baseball.

“He’s done a lot of damage (to the Yankees).”

Maybe Ortiz’s second-biggest achievement – after the championship rings – is that he helped Boston to the top by knocking down the Yankees. Over his career, he batted .303 with 53 home runs against New York.

Of course, the Ortiz farewell tour doesn’t end with Sunday’s game. The playoffs begin for the Red Sox on Thursday.

“Hopefully, there’s another strong run left and he puts it in overdrive,” Farrell said.

Contrast that to other players in their final season before retirement.

Yankees great Derek Jeter, for example, finished his career at Fenway Park, on the last day of the regular season in 2014. New York did not make the playoffs that year and Jeter weighed down the team, batting .256.

And how did Ortiz begin his final regular-season series at Fenway? He hit a game-winning, two-run homer Friday night in Boston’s 5-3 win over Toronto.

PAPI IN THE CLUTCH: PRICELESS

If you look purely to numbers, Ted Williams is Boston’s greatest hitter, and possibly the best hitter the game has ever known.

In 19 seasons, Williams batted .344 (tied for seventh all-time) with 521 home runs, and a .482 on-base percentage (the best all-time).

Imagine Williams’ numbers if he had not lost nearly five seasons because of his service in the military. He served in World War II from 1943-45 (during his prime ages of 24 to 26). He missed all but 43 games over a two-year span (1952-53) because of the Korean War.

Like Ortiz, Williams performed well in his final season, batting .316 with 29 home runs at age 41 in 1960. A left fielder, Williams might have kept on playing if there was a designated hitter back then.

Ortiz’s numbers are impressive – including 541 home runs and a .286 career batting average – but they don’t match Williams’.

Ortiz’s achievements are sometimes downplayed because he has been a designated hitter.

But how do you downplay Ortiz’s bat? Or his clutch performances?

Williams was one of the all-time great players. But even with his talents, Boston never won a championship. Back then, of course, there were no playoffs; the top team in each league went to the World Series.

With Williams, the Red Sox twice finished one game out of first place, in 1948 and 1949. They went to the World Series once, in 1946, losing to the St. Louis Cardinals in seven games. Williams played with a sore elbow and batted only .200 (6-for-30) with no extra-base hits.

Ortiz has played in 82 postseason games, with a .295 average and 17 home runs.

He entered Red Sox lore in 2004, during the American League Championship Series. With the Red Sox trailing the Yankees three games to none, Ortiz recorded two walk-off hits in extra innings – a 12th-inning, two-run homer in Game 4, and a two-out, RBI single in the 14th of Game 5.

In Game 7, Ortiz put Boston ahead with a two-run homer in the first inning as Boston completed the improbable comeback. In the World Series sweep of the Cardinals, he had a home run and four RBI.

The Red Sox won the championship again three years later, sweeping the Colorado Rockies in the World Series. Ortiz batted .370 in the postseason, with three home runs and 10 RBI.

A LEADER IN THE CLUBHOUSE

The amazing part of David Ortiz’s career in Boston is that he began as a part-time player, after being released by the Minnesota Twins (who didn’t want to pay him more money in arbitration). When the Red Sox opened the 2003 season, Jeremy Giambi was the designated hitter.

Two months into the season, Ortiz had played only 31 games. But by June, Ortiz became a regular in the lineup.

Later, it was not only his clutch performances, but his persona that bolstered the Red Sox. He became Big Papi, a leader among his peers and, especially in recent years, a mentor to younger players.

“He just helps you out so much,” said Mookie Betts, 23, who is often seen conversing with Ortiz in the clubhouse.

Before a game, Ortiz will always chat up players from the other team, or be walking in the outfield, his arm around a younger teammate.

“He’s the same with everybody,” said Xander Bogaerts, 24, “inside and outside the clubhouse.”

Despite his heroics in 2004, Ortiz’s greatest season was 2013.

It began with, of all things, a speech. In the first home game after the Boston Marathon bombings, Ortiz spoke briefly during a pregame ceremony, emphasizing pride in Boston, using an expletive to sum up the anger and resilience of the city.

Six months later, the Red Sox faced Detroit in the American League Championship Series.

Boston lost Game 1 and trailed in Game 2, 5-1 in the eighth inning. With two outs and bases loaded, Ortiz launched a grand slam off Joaquin Benoit, the ball landing in Boston’s bullpen, as the Red Sox rallied for a 6-5 win.

In the World Series against St. Louis, the Red Sox trailed two games to one and were tied 1-1 heading into the sixth inning of Game 4. Ortiz called his teammates over for an impromptu talk in the dugout. “We don’t get here every day,” he said. “Let’s relax and play the game we know how.”

Pep talks in baseball can be overrated, but the Red Sox did not lose another game.

Ortiz was named the series MVP after batting .688 (11-of-16) with two doubles, two home runs and eight walks.

This season, Ortiz is coming up huge once again. On Sept. 11, Boston played in Toronto, clinging to a one-game lead in the American League East. The Red Sox trailed by a run in the sixth inning, with two runners on, when Ortiz came to bat to face none other than Benoit. He hit a three-run homer and the Red Sox won, 11-8.

“Benoit on the mound and we need a big hit,” Farrell said, “and he hits a three-run homer. Three years ago, it’s a grand slam.”

And 12 years ago, there were some big blasts to beat the Yankees.

Maybe there is one more month of drama left in the big guy. Boston’s all-time MVP.