BOSTON — Andrew Benintendi added muscle in the offseason. But what still amazes are his nerves of steel.

Benintendi is a 22-year-old rookie. Really, he is.

Pittsburgh’s Gerrit Cole should have had the Red Sox left fielder chasing a curveball for strike three to end the fifth inning.

Benintendi had just fallen behind in the count, 1-2, on a 97-mph fastball. Boston had two runners on base. Benintendi had to be looking to swing, and swing quickly at Cole’s heat.

Cole came with a curveball and Benintendi stayed away for a 2-2 count.

“He has a short track record, we know, but there has never been evidence of panic, even in a two-strike situation,” said Red Sox Manager John Farrell.

Cole stood on the mound and looked in. Catcher Francisco Cervelli put down one finger and Cole gripped the ball for another four-seam fastball. This one came in at 98 mph.

It went out faster. Benintendi applied his Michelangelo swing – a thing of baseball beauty – and swatted it. The exit velocity was 103 mph as Benintendi’s bash sailed into the Pirates’ bullpen.

“His swing is so effortless and smooth, you can’t tell if he ever overswings the bat,” Farrell said.

“For a guy of his stature (5-foot-9), there is such great timing and fluidity to the swing, he creates easy power. And that was the case with that swing.”

That swing produced a three-run homer for a 5-0 lead and the eventual winning runs in Boston’s 5-3 victory Monday on Opening Day at Fenway Park.

And if we can offer this reminder: Benintendi was first promoted last year on May 16 – from Class A to the Portland Sea Dogs. He eventually reached Boston last season.

Now he is batting second in the Red Sox lineup – and producing.

Dustin Pedroia, who knows a little about high rookie expectations, sees a lot in Benintendi.

“The sky’s the limit on his potential,” Pedroia said. “His at-bats are very professional. He always controls the at-bat.

“He’s great in the outfield. He’s instinctive. Everything … it’s right in front of him. It’s a matter of going out and doing it.”

In the outfield, Benintendi may have saved the game in the seventh inning. With the bases loaded and one out, and the Red Sox leading 5-2, Pittsburgh’s Starling Marte smoked a line drive to left-center off Matt Barnes.

Benintendi got a good jump, ran it down and made a leaping catch, changing a multirun, extra-base hit into a sacrifice fly.

“I thought I had a good first step,” Benintendi said. “After that I was trying to time my jump. I was lucky enough to catch it.”

Benintendi sounded almost as nonchalant about his home run.

“Fastball in,” Benintendi remembered. “I thought the wind was going to hold it up a little bit, but I was fortunate enough that it went over.”

And how about the curveball before the home run? Benintendi shrugged. “I don’t remember.”

Red Sox fans will remember Benintendi.

There are so many expectations placed on him. He deflects them as well as other players before him.

Mookie Betts homered in the past two season openers. In 2015, all Betts talked about was the improvements he needed to make.

Ask Benintendi about his success and he smiles.

“You let it go in one ear and out the other,” he said. “It all comes down to playing well and winning.”

If Benintendi stays off the bad pitches and squares up fastballs like he did Monday, the Red Sox have a chance to win.

“He sees the ball extremely well. He’s got a true understanding of the strike zone,” Farrell said.

“Pretty special young player.”

Benintendi is the youngest Red Sox left fielder to start on Opening Day in 55 years. Carl Yastrzemski was 40 days younger than Benintendi was when he started in left in 1962.

Too early to make those kind of comparisons.

But, geez, the kid can swing the bat.

Kevin Thomas can be reached at 791-6411 or:

[email protected]

Twitter: @ClearTheBases