TORONTO — CC Sabathia’s face, which often gives away as much as the best poker players’, told plenty.

The lefthander was pulled after three innings Tuesday night, having allowed a pair of homers to Josh Donaldson in the Yankees’ 4-2 loss to the Blue Jays in front of 41,596 at Rogers Centre.

More significant than the defeat, which dropped the Yankees four games behind the Red Sox in the AL East, was the reason the veteran lefthander was pulled.

A right knee that has caused Sabathia trouble in the past — surgery on it ended his 2014 season and he had what the club called a “cleanup procedure” on it last October — flared up.

“It’s just frustrating,” said Sabathia, who used the word “pain” to describe what he felt throughout the three innings in which he allowed four runs and six hits. “It’s been healthy all year. To go out today and not have it be where I want it to be is just frustrating.”

Joe Girardi said Sabathia, who very well could be DL-bound, would be sent back to New York for further evaluation.

“There’s concern there,” the manager said.

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A glum-looking Sabathia said that he couldn’t point to one pitch when he felt it.

“It’s kind of out of nowhere,” he said.

Sabathia (9-5, 4.05 ERA), who had a 3.44 ERA this season as recently as July 21, said, “It’s just hard to land. It’s hard for me to finish my pitches. It’s just hard for me to get over my front side and finish the pitches. When I can’t do that, I don’t know where the ball is going.”

If Sabathia ends up on the DL, rookie lefthander Jordan Montgomery, optioned to Triple-A Scranton/Wilkes-Barre Sunday night, would be a likely candidate to take his place.

“He’s [Sabathia] a very important piece to this team,” said Gary Sanchez, who took a 95-mph Ryan Tepera fastball off his left wrist in the eighth inning but said “it’s fine” afterward. “He’s important to us.”

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The Yankees (59-52), coming off a 2-2 split in Cleveland in which they won the final two games, managed seven hits against five pitchers in losing the first of three games against the Blue Jays (53-59).

After Sabathia, who brought an AL-best 2.29 road ERA into the night, departed, righthander Bryan Mitchell, called up earlier in the day, threw four scoreless innings to give the offense a chance to rally.

It did not.

Toronto lefthander J.A. Happ (5-8, 3.77) allowed one run, four hits and four walks in 5 2⁄3 innings. The Yankees went 2-for-7 with runners in scoring position and stranded 10.

The Yankees loaded the bases with one out in the eighth against Tepera, who hit two batters and walked another, but managed just a Garrett Cooper sacrifice fly. Ronald Torreyes flied to center to end the threat.

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Toronto closer Roberto Osuna retired Brett Gardner, pinch hitter Jacoby Ellsbury and Aaron Judge in a perfect ninth for his 29th save. Judge went 0-for-3 with a walk and two strikeouts, making it 25 straight games in which the rookie has struck out, extending his franchise record.

“I thought the at-bats were pretty good,” Girardi said. “I thought Coop just missed a ball in the eighth inning, ended up driving in a run, but he just missed it. We just never put the hits together when we needed them tonight and that was the difference.”