BOSTON — With four-fifths of the starting rotation from the beginning of the season traded away, the Boston Red Sox intend to use the final two months of the season to take long looks at their youngsters.

The downside of that strategy was on display Saturday.

Staked to an early three-run lead, Allen Webster imploded, walking five batters in a four-run third inning as the New York Yankees rallied for a 6-4 victory to spoil outfielder Yoenis Cespedes’ Boston debut.

“It was clear he lost command of the strike zone,” Red Sox Manager John Farrell said. “And while there’s plenty of stuff in terms of fastball action, swing and miss to his change-up, the ability to make adjustment whether pitch to pitch or hitter to hitter was elusive.”

In his fifth career major league start and his first at Fenway Park, Webster (1-1) said he lost his release point. He walked the bases loaded to start the third before Derek Jeter’s two-run bloop double to right.

Jacoby Ellsbury and Carlos Beltran drove in runs sprinkled between two more walks before Farrell lifted Webster.

Webster threw only 39 of 71 pitches for strikes, giving up four runs and six walks.

“It’s real tough,” he said.

Welcome to the second half of the Red Sox season, with tough lessons for all.

“There’s a drastic difference between here and Triple-A,” Farrell said.

Cespedes declared through an interpreter, “I like it here,” after receiving loud cheers as he stepped into the batter’s box to lead off the second inning. It was two days after he was acquired from Oakland for Jon Lester in a flurry of moves.

Cespedes went after a low, off-speed pitch and pulled it into left field for a single in his first at-bat. But he finished 1 for 4, including a pop out against hard-throwing right-hander Dellin Betances with a runner on second to end the seventh.

Cespedes played left field although the Red Sox plan to move him to right after he gets more practice time at Fenway Park.

“I haven’t really played much right field,” Cespedes said. “But in my opinion, good outfielders should pride themselves on being able to play all three positions and that’s what I’m going to do.”

Mike Napoli’s two-run homer to left off starter Shane Greene helped Boston take a 3-0 lead in the second. But reliever Shawn Kelley (2-3) struck out Napoli with two on to end the fifth and the only other Boston run came on David Ortiz’s sacrifice fly in the seventh.

BRADLEY’S GLOVE

Jackie Bradley Jr. entered the day tied with his new teammate, Cespedes, for the AL lead with 12 outfield assists.

Bradley took over the top spot when he doubled Jeter off first base on a fly ball to end the first.

It was Bradley’s eighth double play of the season, the most by a Boston outfielder since Dwight Evans had eight in 1975. The last Red Sox center fielder with eight double plays was Nemo Leibold in 1921.

TRAINER’S ROOM

Boston catcher David Ross was placed on the 15-day disabled list with plantar fasciitis in his right foot. … Outfielder Shane Victorino, on the 15-day DL with a strained lower back, is expected to see an orthopedic surgeon in Los Angeles early this week. … Infielder Kelly Johnson is expected to play two rehab games later this week.