BOSTON -- Boston's lead seemed shaky when the Detroit Tigers loaded the bases with the go-ahead run at the plate.

Then the Red Sox were saved by the rain.

The tarp came on the field at that point with two outs in the sixth inning, the game was called 1 hour, 45 minutes later, and the Red Sox had a 4-1 win on Tuesday night, their fourth in a row.

"Unique," Boston manager Bobby Valentine said of the bases-loaded threat that was washed away. "I don't think it's ever happened before."

The game began in light rain that got heavier. Even Detroit manager Jim Leyland supported the decision to halt it even though his team had a shot at a big inning but ended up with its fifth loss in six games.

"The umpires do the best they can with that stuff. We had been playing in some pretty heavy rain," he said. "They definitely made the right call calling it now. So be it. We put ourselves in an unfortunate situation."

Clayton Mortensen (1-0) made the most of his strange situation, pitching 2 2/3 shutout innings in relief of injured starter Josh Beckett after being recalled from the minors earlier Tuesday.

He was in Rochester with Pawtucket for a Triple-A game when he was told to get to Fenway Park. He arrived less than two hours before the game then was thrust into action when Beckett suffered a spasm in his lower back and left with two outs in the third.

"It was kind of unexpected, I guess, but being a reliever you've got to be ready at any given point," Mortensen said. "Something crazy can happen. It wasn't the most ideal situation I wanted to come into a game, but I'll take it."

He very well could be back with Pawtucket on Wednesday once lefty Craig Breslow, obtained in a trade with Arizona on Tuesday, reports to the Red Sox.

"I completely understand my role," said Mortensen, who was thrilled with the fans' cheers as he left the game. "I almost couldn't help smiling. ... It was probably one of the coolest things I've ever experienced."

Mortensen allowed one hit and three walks with one strikeout. Franklin Morales retired one of the three batters he faced but got his first save of the year.

Tigers ace Justin Verlander (11-7) gave up all four runs in the fourth inning when he issued two of his four walks.

"I was able to keep my hand pretty dry," he said. "Once I tried to start throwing a little bit harder, naturally your stride gets a little bit longer so I started landing in an area that I wasn't landing in before, so I think I started slipping a little bit. ... It's the same for both sides."

Detroit scored in the first when Miguel Cabrera walked with the bases loaded on Beckett's last pitch of the game.

The struggling right-hander had been the subject of trade speculation before Tuesday's 4 p.m. deadline for non-waiver deals. He began the day 5-9 with a 4.57 ERA and a 1-5 mark in his previous nine starts.

Subpar seasons by Beckett and Jon Lester, who began 2012 as Boston's top two starters, are two key reasons for a mediocre year in which the Red Sox have hovered near the .500 mark.

But Beckett looked like a different pitcher at the start on Tuesday.

He retired the first eight batters then got Omar Infante to hit a weak grounder to shortstop. Pedro Ciriaco charged the ball, but Infante beat his throw to first. Then Beckett fell apart, hitting Austin Jackson with a pitch and walking Quintin Berry and Cabrera.

Beckett then motioned to the dugout and the trainer came to the mound. He checked the left side of Beckett's lower back and the pitcher walked slowly off the field.

Beckett pitched well out of the windup, but once Detroit got a runner he had trouble from the stretch.

"The footing just wasn't the same and his back spasmed up a little," said Valentine, who described Beckett's condition as day to day.

Verlander, who had gone at least six innings in his previous 63 starts, gave up two hits through three innings before struggling as the rain became heavier in the fourth.

"Kind of a weird way for (the streak) to come to an end," he said. "I get a five-inning complete game but I don't go six innings."

Cody Ross and Jarrod Saltalamacchia singled in the fourth, and, after Will Middlebrooks struck out, Kelly Shoppach walked to load the bases. Ciriaco tied the game with a lined single to right and Jacoby Ellsbury forced in the go-ahead run with a walk.

Carl Crawford then grounded a single that trickled by second baseman Infante and drove in Shoppach. And when Infante was slow retrieving the ball, Ciriaco also scored.

The Red Sox got another break in the sixth when the rain ruined an excellent opportunity for the Tigers.

Fielder led off with a walk and Delmon Young flied out. Morales replaced Mortensen and Fielder was forced out at second on Ryan Raburn's grounder. But Alex Avila singled and, with the rain pouring, Jhonny Peralta walked.

And then the tarp came out.

Would Valentine liked to see a rule change that allows such games to be continued the next day?

"Not when we're leading," he said. "I think this is a great rule."

Game notes

The Red Sox obtained Breslow from Arizona for RHP Matt Albers and minor-league OF Scott Podsednik. Breslow was not with the Red Sox for the game, and Mortensen was brought up to replace Albers on the roster. ... OF Ryan Sweeney was put on the 15-day disabled list after undergoing surgery Tuesday for a fractured knuckle on his left pinkie. OF Ryan Kalish was recalled from Pawtucket. ... Verlander was 6-2 with a 2.47 ERA in his previous eight starts. ... Boston is 2-0 on its season-long 10-game homestand.