Suddenly, it was 2016 all over again for Luis Severino.

A lack of focus led to a terrible start Saturday as the Yankees were pummeled 10-5 by the Red Sox, stunning a Yankee Stadium crowd of 47,241. The pitching poor Yankees could not build off Friday night’s 5-4 comeback victory over Boston as they slipped 4 ¹/₂ games back of the Red Sox in the AL East.

With Masahiro Tanaka going on the disabled list earlier in the day the Yankees needed a big outing from Severino, and got a pitcher who lost his focus and could not do well against the Yankees’ chief rival.

The Yankees most reliable starter was lost on the mound, as a result the Yankees were dumbfounded. Sure, Todd Frazier’s error to load the bases in the third didn’t help, but moments after that, Severino allowed a two-run single to left to Mookie Betts and never bothered to back up home plate, staying near the mound.

Then to the next batter Andrew Benintendi, he allowed a scorching three-run home run to right field, a 98 mph fastball that Little Ben crushed, hitting the ball off the top of the wall in right and skimming into the seats.

This home run was not to be confused with the three-run bomb Benintendi hit in the fifth up into the second level of the right-field bleachers. That home run came off a Severino crush-me slider.

That third inning began harmlessly enough with two one-out walks issued by Severino to the eighth and ninth hitters in the Red Sox order. The first was a nine-pitch walk to catcher Christian Vazquez and it was all downhill from there for Severino. That walk seemed to take a lot out of him.

When the dust cleared, Severino lasted one out into the fifth inning, allowing 10 runs, eight earned, and those two home runs to Benintendi. It was ugly — 2016 ugly.

The 10 runs allowed by Severino were the most ever allowed by a Yankee pitcher at the current Yankee Stadium.

You’ll remember that in 2016 Severino was 0-8 as a starting pitcher with an 8.50 ERA in 11 starts. Even his worst start in 2016, though, (seven earned runs twice) was not as bad as Saturday’s slaughterhouse performance.

It was a psychological blow to the Yankees as well, having their most successful starter battered at home by Boston.

“I think I was trying to pitch too perfect,’’ Severino said of his bad day.

Severino came into the day with a 9-4 record and a 2.91 ERA. By the end of the long day, he had another loss under his belt and his ERA had ballooned to 3.32.

Severino is just 23. This is the time to learn. This was the important lesson on this day, his 23rd start of the season. In the AL East you cannot lose your guard for even one inning, if you do, you will be in big trouble.

To make it worse, Severino was coming off his best start of the year, allowing only two hits in the Yankees’ 8-1 win Sunday over the Indians in Cleveland. That was Severino’s fourth straight win. Severino struck out only four batters, his lowest strikeout total since a 7-1 loss to the Blue Jays in Toronto when he struck out only three.

Severino walked just two batters, but they both scored in that third inning.

Here is a much more disturbing trend. Severino is 1-4 against the Red Sox with a 5.05 ERA. The Red Sox have his number. Clearly Severino is going to have to make a major adjustment against them.

The Red Sox also did a terrific job of hitting against the Yankees’ shift, imagine that, which may tell you that the Red Sox have the Yankees’ number in that department, too.

All in all, for a team with starting pitching woes, the Yankees are in deep trouble, 60 feet, 6 inches from home plate. With eight games against the Red Sox and Mets these next eight days, the Yankees will have to find a way to overcome their problems.

For Severino it has to start with the right-hander keeping his focus.