BOSTON — Brett Gardner finally landed on the 10-day injured list on Thursday after being out with left knee soreness since Sunday.

Though Aaron Boone said he still considered Gardner “day-to-day” and the Yankees brought up left-hander Stephen Tarpley to take his spot on the roster for Thursday’s 19-3 loss in the series opener against the Red Sox at Fenway Park, the moves left them with just three outfielders.

The injury could result in the return of Clint Frazier.

The manager wasn’t sure who he would turn to if he needed another outfielder, with Aaron Judge, Aaron Hicks and Mike Tauchman the only ones available for Thursday’s game.

“Um, I don’t know yet,’’ Boone said when asked about an emergency outfielder. “We’re kind of working through that, too. … We’ll figure it out. We’ll figure something out.”

Gio Urshela ended up entering in left in the lopsided game in the eighth.

It was Urshela’s first game in the outfield — in his life — and he still made a fine catch in foul territory on a Rafael Devers fly ball and managed to snag Andrew Benintendi’s hard liner for the second out, both with Austin Romine on the mound.

Thursday’s priority, though, was getting another arm up from Triple-A Scranton/Wilkes-Barre to aid a bullpen that continues to be worked heavily during their three slugfests in Minnesota, and Tarpley was needed thanks to Masahiro Tanaka getting knocked out after just 3 ¹/₃ innings.

After the loss, Boone hinted the Yankees could call up an outfielder on Friday.

Cameron Maybin is a possibility to rejoin the roster. He is coming back from a left calf strain and served as the DH against Columbus on Thursday. Maybin went 2-for-6 with a pair of strikeouts and was scheduled to play all nine innings in the outfield for SWB on Friday.

“Potentially he’s an option pretty soon here,’’ Boone said of Maybin, who is on the IL along with Giancarlo Stanton, Gardner and — of course — Jacoby Ellsbury.

That leaves Frazier, who didn’t play in Thursday’s game for SWB, but has been on a tear at the plate recently. He’s 8-for-13 with a double and three homers in his last three games in the minors.

Boone said the reports on Frazier have been “good,’’ adding, “He’s been swinging the bat, too. He was struggling with his swing for a while. This past week, he’s had some really big games down there.”

Frazier is “a consideration if we needed something,” Boone said.

It has been an odd season for Frazier, who has hit well when given the opportunity in The Bronx, but his defense has been inconsistent at best- and abysmal at worst.

He was not shy about expressing his surprise when he was optioned last month upon the arrival of Edwin Encarnacion and made headlines with some of his reactions after his defensive lapses.

Frazier didn’t help his cause by not hitting when he first went down to Triple-A and he could still be a part of a transaction before next week’s trade deadline, as the Yankees continue to get production out of Tauchman.

As for Gardner, he acknowledged the team couldn’t wait any longer for him to return to action without going to the IL and he’s hoping a few more days off does the trick.

Gardner aggravated his sore knee when he made a diving catch Saturday at Yankee Stadium and has been out since. Boone said he thought Gardner could be back Tuesday when the Yankees return home.

“I hope so,’’ Gardner said. “That’s the plan.”