PHOENIX — How is this Boston Red Sox starting rotation going to get back on track after beginning the season with an MLB-worst 9.13 ERA?

It could be as simple as continuing to build arm strength, increasing velocity and rediscovering power.

That might take a few more weeks.

Ask Brian Bannister what he sees with all the opposition home runs and the answer is simple: It’s all about stuff. And the Red Sox need to rediscover their stuff.

“Our pitchers will continue to get back to their peak velocities and you’ll see improved production as that happens,” said Bannister, the team’s assistant pitching coach known for his understanding of pitch data.

Bannister estimates it takes about six weeks for a pitcher to regain his full firing capacity in a new season. Six weeks is usually the duration of spring training, but the starting pitchers weren’t exactly letting it rip this spring.

Most didn’t see game action until early March, just three weeks before Opening Day.

Chris Sale’s spring debut wasn’t until March 16. He was throwing in only controlled environments until then. Six weeks from that date would put him near the beginning of May before he’s at his strongest.

David Price made his spring debut March 12, Nathan Eovaldi on March 13 and Rick Porcello on March 10. Eduardo Rodriguez had the earliest start – Feb. 27.

It’s all part of the grander plan to keep them throwing harder later in the season. It worked fine last year but seems to have come at a cost this time.

Sale, Eovaldi and Rodriguez have all seen decreases in their average four-seam velocity from last year, with Sale’s 91.3 mph down from 95.6 mph the most drastic. Eovaldi is averaging 97.4 mph after averaging 99 mph last October.

Rodriguez is averaging 92.6 mph after averaging 94.1 mph last October.

Pitching coach Dana LeVangie said of Sale last week, “You guys want him to pitch the whole year or do you want him to go out and throw 100 mph right now and not be there for his team? He’s building.”

The results haven’t been good. Here are the five starters’ slugging percentages they’re allowing on their four-seam fastball this year compared to last year: Sale (2.000 vs. .339), Eovaldi (.400 vs. .309), Rodriguez (1.059 vs. .381), Porcello (.556 vs. .437) and Price (1.000 vs. .461).

Think of a golf swing. As a golfer tries to hit the ball harder, often that comes at the sacrifice of accuracy. Pitching is similar.

As the power returns, Red Sox starters won’t have to dig back and try harder to reach their average velocities, a mechanism that can result in too many misfires and often bad location that’s the cause of a lot of home runs.

Together the top five Red Sox starters have allowed 16 homers in 10 starts, easily the worst in baseball.

Bannister explained what he’s seeing from opposing hitters’ approach.

“The goal of every hitter right now is to … hit the ball in the air,” he said. “And (that) also gives you more margin for error as a hitter to handle the steadily increasing velocity across baseball.

“Whether we like it or not on the pitching side, velocity is extremely important. You see once again secondary pitches handily outperforming fastballs across the league. So there’s a certain level of velocity you need to attain as a pitcher, and maintain it. And then you have to be able to get hitters to generate swings and misses. Hitters are completely content with an all-or-nothing approach right now. I think something like roughly like 40 percent of all runs right now are home runs. That’s just the reality of modern pitching.”

Through Sunday, 46 percent of the runs in the majors this season were scored on home runs. That number was 42 percent last year.

Overall, home runs are up 9 percent from one year ago.

“You have to have velocity, you have to train for velocity and you have to be able to throw secondary pitches,” Bannister said. “That’s a mandatory requirement of all pitchers in today’s game, where that wasn’t the case 10 years ago, where hitters were trying to put the ball in play with two strikes, or move the runner over, or any of these things that are no longer part of today’s hitting philosophies with the shifts and advanced defenses.

“It’s velocity and spin, or die.”

While the Red Sox did question the game plan for Rodriguez in his first start, it’s also easy to point to his diminished fastball velocity to understand why he hasn’t had much confidence in the pitch this year.

Overall the Red Sox are OK with their pitching strategy, which last year was summed up by fastballs up in the zone and breaking balls down.

“Nothing has changed in that department,” Bannister said. “It’s just with every year, pitchers are throwing harder. Teams are finding a way to make the average pitch be faster, whether it’s the use of an opener, more relievers, less pitches per outing for a starter, as well as more advanced training methods in the offseason and during the season. So there’s just a very strong velocity component to everything that’s done in pitching.”

The Red Sox are hoping to see their pitchers’ velocity increase soon.

“The other thing that happens as a side effect of that is only the hitters that can handle velocity remain at the major league level,” Bannister said. “The guys that can’t handle velocity, through reaction time or eyesight or hand quickness, they’re naturally filtered out of the major leagues. And the only guys left over year after year are the guys that can handle velocity. So it just makes it even that more important that pitchers throw hard, but also have other ways to complement velocity.”

To Bannister, it all boils down to stuff.

“It’s the reality of today’s game,” he said. “You either train for stuff, go out and execute stuff, or it’s very hard to pitch right now.”

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