BOSTON — The Arizona Diamondbacks will pit their ace, Zack Greinke, against the Boston Red Sox on Sunday. It will be an intriguing matchup.

But Boston isn’t using its supposed ace. David Price pitched Friday.

And All-Star pitcher Steven Wright is out with a sore shoulder.

No ace. No All-Star. Boston simply will use its best pitcher, Rick Porcello.

Yes, Price has more strikeouts (167 to 125), and Wright offers a lower ERA (3.01 to 3.40).

But Porcello gives something this erratic Red Sox club needs desperately – consistency.

And did we mention wins? Porcello’s 15-3 record is second in the majors behind J.A. Happ of Toronto. Porcello is putting up career-best numbers in almost every category, showing the promise Boston believed in when it traded for Porcello, then signed him to an $82.5 million extension.

“Just executing pitches,” Porcello said after his latest start, an 8-inning, 2-earned run effort in a win against the Yankees. “Making good pitches at the right time.”

Once upon a time, many of those pitches weren’t so good and the timing was lousy. That was halfway into the 2015 season, when the Porcello trade and his contract extension looked disastrous … as disastrous as the season itself.

Last year’s rotation was portrayed as a deep group but without an ace. But eyes looked to Porcello to assume the role, even though the injury-prone Clay Buchholz was the opening-day starter.

Porcello came to Boston from Detroit for Yoenis Cespedes, an outfielder the Red Sox acquired when they traded Jon Lester to Oakland. Fair or not, the perception is Porcello was replacing Lester.

Then there was the contract. Porcello was scheduled to be a free agent after the 2015 season. Afraid to get into a bidding war, Boston locked him into a four-year extension through 2019.

Boston committed over $20 million per season for a pitcher who was never one of Detroit’s top starters. In the Tigers’ four playoff appearances, from 2011-14, Porcello had eight appearances and only two starts.

Porcello isn’t considered an ace pitcher but seemed to shoulder that responsibility, especially with his contract and Boston’s ace-less rotation.

In 2015, as Boston was free-falling to another last-place finish, Porcello was 5-11 with a 5.81 ERA through his July 29 start. He then went on the disabled list with a triceps strain.

The break seemed to do him good. After he returned Aug. 26, Porcello went 4-4 with a 3.57 ERA.

This season the pressure was removed when the Red Sox signed Price, whose seven-year, $217 million contract made Porcello’s deal seem a bargain.

But Porcello has been just as valuable as Price. Porcello leads the team in wins and WHIP (1.08). He is second to Wright in ERA and batting average against (.227 to .239).

Price has pitched more innings (1632/3 to 151) but with two more starts (25 to 23).

While Porcello doesn’t blow up the radar gun – a 92-93 mph four-seam fastball and an 89-90 two-seamer (sinker), Porcello mixes in a slider, curve and change-up. He’s striking out a greater percentage of batters than ever (20.4 percent) and walking a fewer percentage (4.2 percent).

“He’s doing a great job locating pitches,” catcher Sandy Leon said.

And Porcello has been dependable all year. He’s allowed more than three earned runs six times – but five of those times were four earned runs, and the other five earned runs. His worst month was May (4.65 ERA) and he’s been pretty spot-on since.

“He gets to that middle portion of the ballgame and really begins to settle in,” said Manager John Farrell, who was especially pleased with the Yankees start. “I thought he had a very good feel for his change-up and his curveball … the sinker in the bottom of the zone. Just an outstanding effort.”

Price has allowed over three earned runs eight times, including three six-run starts, and one allowing eight earned runs.

Wright has given up over three earned runs four times, one of them eight earned runs. Wright has scuffled some lately, with a July ERA of 6.23. He had a great start to begin August before the shoulder injury.

The difference in Porcello this year is seen in the numbers, and in both his comfort and confidence.

“There’s definitely much more of a comfort level here in all aspects,” Porcello said. “Just with that, I’m trying to go forward and do my job and help us achieve our goals.”

Those goals obviously include the postseason. Boston is still in the race, although hardly playing inspiring baseball. In this stretch run, Porcello should get between nine and 10 starts.

“There’s an urgency to win and we have to play with an urgency,” Porcello said. “I don’t try and let that affect me while I’m out there, but it’s something we all know.”

And the Red Sox know their winningest pitcher takes the mound Sunday.