BOSTON -- With the Red Sox on the brink of being swept at home for the first time since April, the presence of pitcher James Paxton, who was coming off a winning performance in his last outing, was enough to provide some hope for the weekend.

But just five pitches into his day, the Red Sox starter would be forced to leave Sunday's series finale against the Astros with a right calf strain, forcing the club to go to the bullpen early in an eventual 10-2 loss at Fenway Park.

“It just felt like I got kicked in the calf,” the hobbled Paxton said postgame after making it through just three batters on the day. “Then I kind of turned around, took another step and felt it again, and I was like, 'Oh, it’s not good.'”

The left-hander looked in control early, allowing a leadoff single to Jose Altuve before forcing Alex Bregman into a double play for two outs on just two pitches.

Then Yordan Alvarez stepped to the plate and hit a sharp grounder to Dominic Smith at first base.

With Smith unable to cleanly field the ball, Paxton broke off the mound to cover the first-base bag, but he instead came up limping after his first few steps and was only able to take a few more before kneeling on the infield grass in noticeable pain.

“I felt it pop,” the Sox starter said.

“He’s going to have an MRI tomorrow,” said manager Alex Cora. “We’ll see where we’re at. Tough one. He’s been healthy the whole season and then that happens.”

The Red Sox training staff and coaches came out to check on their starter, and the left-hander was helped off the field soon after, unable to bear weight on his leg.

Sunday marked Paxton’s third start since rejoining the Red Sox in a trade with the Dodgers. Overall this season, the veteran southpaw is 9-3 with a 4.40 ERA in 21 starts.

Lucas Sims came on in relief and began what was ostensibly a second straight bullpen game.

The Red Sox used four pitchers in Saturday’s 5-4 loss and threw another five at the problem on Sunday, including Paxton.

When that proved not enough to get through nine, Cora put first baseman Smith on the hill to pitch the top of the ninth.

The Astros got four off Brennan Bernardino in the fifth inning, including back-to-back home runs from Bregman and Alvarez, and then they tacked on five more off Luis García in his two innings.

“I’m just not making pitches, and some of the pitches I have made have been hit,” a forlorn Bernardino said. “It’s just not going my way right now.”

“It’s not going the way I would expect it,” added García of his efforts since joining the club at the Trade Deadline. “The only thing I have to do is get better and help the team the best I can.”

Paxton’s acquisition provided the Boston rotation with a much-needed veteran arm in the mix, but with the hard-luck lefty sounding as though he might be shelved again, a wrench has been thrown in the Red Sox plans right in the midst of a run of 13 straight games without an off-day.

“I was feeling really good,” said Paxton. “I was really looking forward to helping this team go where they want to. Hopefully, I can come back here at some point before the end. We haven’t gotten any imaging yet, so we don't know what the severity is, so we’ll see. It hurts.”

“We’re good coverage-wise unless something bad happens, and it happened,” said Cora of his thought process prior to his Sunday starter going down. “The boys battled today. Guys pitching in different roles and extended them. We had to somehow, some way, get 27 outs. Obviously, the score is the score, but we were in survival mode from the get-go.”