Breaking down which parts of Red Sox winning streak are sustainable vs. questionable

Tomase: Which parts of Red Sox' win streak are actually sustainable? originally appeared on NBC Sports Boston

After ripping off eight straight wins to move out of last place in the American League East and into wild card position, the Red Sox have positioned themselves to play meaningful baseball this summer.

With the sixth-best record in baseball, they have already exceeded expectations, which leads to a question: How much of what they're doing is sustainable, and what areas remain causes for concern?

Let's break them down.

Sustainable: Masataka Yoshida

Every hitter who arrives from Japan wonders if his power will translate to the higher velocity and bigger ballparks of the big leagues. Yoshida was no exception, and the early returns were not encouraging. Halfway through April, he was pounding balls into the ground at an unsightly 70 percent rate. But since opening his stance, he has caught fire.

Yoshida brings a 16-game hitting streak into Tuesday's meeting with the Braves, and it's not a fluke. He is hammering everything from foul line to foul line, batting .324 with six homers and 24 RBIs. During his hitting streak, he has nearly as many home runs (5) as strikeouts (6), and fastballs haven't been a problem. He's hitting them at a.366 clip and there doesn't appear to be a hole in his swing.

The Red Sox were quietly mocked by other organizations for giving Yoshida $90 million, but they appear well on their way to getting the last laugh.

Questionable: Starting rotation

Only three teams own worse starting ERAs than Boston's 5.89, and they're a combined 42 games below .500. The Red Sox also rank among the bottom five in quality starts, which is taxing their otherwise tremendous bullpen.

It's unclear how this will shake out, though James Paxton's return from a parade of injuries could help. Veteran Chris Sale and youngster Brayan Bello have pitched better of late, giving the top of the rotation some potential definition. But too many questions remain.

Is the irritated nerve in Garrett Whitlock's elbow no big deal or a legitimate cause for concern? Can Tanner Houck learn how to turn over a lineup more than once? Is Nick Pivetta anything better than slightly below average? Does veteran Corey Kluber have another gear?

It's hard to envision the Red Sox contending long-term with their starters barely averaging five innings a game. The bullpen won't last. Which leads to...

Sustainable/Questionable: Bullpen performance/attrition

The individual arms have been tremendous. Closer Kenley Jansen already looks like one of the most impactful signings of the offseason. Setup man Josh Winckowski has come into his own as a multi-inning weapon. Chris Martin has pitched well save for one hiccup in Tampa.

Chief baseball officer Chaim Bloom prioritized strike throwing, and the new-look bullpen has delivered, with the fifth-fewest walks in relief despite throwing the fifth-most innings. Sox relievers are 8-1 with a 3.46 ERA and a league-low two blown saves.

But we're already seeing cracks. Martin just returned from a stint on the injured list. Workhorse swingman Kutter Crawford replaced him with a sore hamstring. Jansen recently missed a few days with a sore back. Right-hander Zack Kelly is on the 60-day IL. John Schreiber's velocity is down a tick.

Asking them to keep producing this effectively at this volume without further damage is a recipe for disaster. The starters must do more.


Sustainable: Offensive approach

In a game that suddenly values the ability to put the ball in play again thanks to a ban on shifts, few teams move the chains better than the Red Sox. Though they rank third in the American League in home runs while boasting AL leader Rafael Devers (11), they're more defined by their ability to create traffic.

Their .271 team batting average trails only the Rays, and is 11 points higher than their average at this time last year, which also ranked second in the AL. They don't strike out, they're the most efficient base stealers in the game (23 for 25), and they've scored more runs with two strikes than everyone except the Rangers and Rays.

Their lineup is frequently described as relentless, from Alex Verdugo to Yoshida to Devers to veteran Justin Turner. They may slump, but their ability to put bat to ball means they'll be better positioned to minimize any dry spells.

Questionable: Jarren Duran

The young center fielder has been a revelation since finally finding a comfortable batting stance that allows him to reach elevated fastballs. He leads the team in average (.366) and OPS (1.018), and it's safe to say neither of those numbers is sustainable.

The question is how Duran responds to an inevitable slump. He has been his own worst enemy in the past, and while his strikeout rate has dropped in each of the last three years, it's still over 25 percent.

Opponents have now had three weeks to get a read on him. We'll see how they attack him and how he responds.

Sustainable: The vibe

The Red Sox poisoned their own clubhouse last year by lowballing the popular Xander Bogaerts, a move that created wariness of the front office. Recognizing the need to rebuild trust, they signed a number of high-character veterans with World Series experience, from Turner to Jansen to center fielder Adam Duvall.

Sale's return to active duty has added another leadership voice. The result is a professional and quietly confident group that's refreshingly free of drama. Their fate will ultimately be decided by talent, but at least they're less likely to be taken down from within.