BOSTON — When your days are numbered, you have to make the most of them. Tyler Austin did exactly that Saturday night.

With Starlin Castro, Greg Bird and Matt Holliday all close to returning from the disabled list, the recently promoted Yankees outfielder/first baseman very likely is headed to the minors. He’ll return to the Yankees when rosters expand Sept. 1. But when he goes back to Triple-A, he can continue to savor what he contributed in Saturday night’s 4-3 win over the Red Sox at Fenway Park: a three-run homer off Boston ace Chris Sale in his first career at-bat against the lefthander.

“It was a special moment,” Austin said.

With one out and two on in the second inning, Austin unloaded on a 96-mph four-seam fastball from the AL Cy Young Award favorite. His blast cleared everything in leftfield and landed on Lansdowne Street.

“He started me off with two changeups, and all night against him, I was looking for a fastball and didn’t really think too much about breaking pitches,” Austin said. “Just try to sit on the fastball because his breaking pitches are really, really good [and] I didn’t want to get into that.”

With one on and one out in the fourth, Sale again tried to get Austin with a four-seamer. He hit a rocket to centerfield that appeared to be an extra-base hit, but Jackie Bradley Jr. made the catch at the top of his leap against the garage door on the leftfield side of the 420-foot mark.

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“I thought I hit it pretty good. It was a heck of a play,” Austin said. “That was a great play. He’s a really good centerfielder and he’s going to do that to guys for a long time, like the one he took away from [Aaron] Judge earlier this year [robbing him of a home run].”

Austin, who also doubled to left-center to start the ninth, has had a difficult season. He started the season on the 60-day disabled list after fouling a ball off his left ankle and fracturing it in spring training. He returned in late June and played in four games before being sent back to Triple-A Scranton/Wilkes-Barre.

When first baseman Garrett Cooper suffered a hamstring injury last week, Austin was brought up to start against the Mets on Thursday night and went 2-for-4.

Bird (bone bruise in right ankle) followed a two- homer game for Scranton with another two-hit outing Saturday night and could be back with the Yankees as soon as this week’s series in Detroit.

Castro and Holliday both started minor-league rehab assignments on Friday.