BOSTON — In another city, or perhaps another season, he would be the only reason to keep watching, to remain interested in a team already out of contention.

Here in Boston, of course, fans alternate between setting the postseason starting rotation and worrying if a seemingly impenetrable 10½ game lead is sufficient.

But while waiting for the Red Sox’ offense to resuscitate itself, Jacoby Ellsbury is a coming attraction, a sneak preview of the future.

In all likelihood, he won’t be here much beyond the All-Star break. For the time being, he’s little more than a tease. But three days into his major-league career, Ellsbury has brought an undeniable buzz to Fenway Park.

It began Saturday night when, in his second major-league at-bat, he hit a routine roller to short. With a burst of speed down the line, he arrived at the bag a millisecond before the throw from shortstop Michael Young, continuing past the base and onto the outfield grass, like a sprinter after breaking the tape.

Young, an All-Star, looked into the home dugout and made eye contact with Red Sox manager Terry Francona, who served as a Rangers coach in 2002.

What was that?

That was quite likely the fastest player every to wear a Red Sox uniform. The Sox have clocked him going from home to first in 3.9 seconds, the kind of time usually associated with speeding bullets like Ichiro Suzuki.

Before last night’s Red Sox win over Texas, Ellsbury spoke matter-of-factly about his eye-popping speed.

“It gets me on base,” he said. “Infielders have a tendency to rush their throws.”

Making his third start last night, subbing for the injured Coco Crisp, Ellsbury again electrified Fenway.

He notched his second major-league hit, similar to his first, in the third inning. Hitting a chopper between the mound and first, Ellsbury sped toward first as second baseman Desi Relaford and pitcher Brandon McCarthy converged, fully aware of who was running and the need to get the ball to first as quickly as possible.

It didn’t matter as Ellsbury reached safely.

After another single to right in the fourth, his first out of the infield, Ellsbury put on his purest display of speed yet. He stole second for another major-league first, then broke from the bag when McCarthy skipped a wild pitch past catcher Gerald Laird.

When Ellsbury saw that the ball had rolled toward the visitors’ dugout, he never slowed as he approached third and seemed to find another gear as he sprinted for home. He finished with an artful hook slide, ultimately unnecessary, as Laird’s throw to McCarthy wasn’t close to being on time.

The crowd of 36,778 stood as one and roared its approval.

In the dugout, Francona watched with admiration.

“We already, obviously, knew he runs well,” said Francona, “but that was a treat to watch. His speed is fun, but his instincts are impressive for a young player.”

Francona joked that third-base coach DeMarlo Hale may have to make the biggest adjustment of anyone in a Red Sox uniform. Hale is accustomed to station-to-station baserunners, not locomotives like Ellsbury.

“I saw DeMarlo wave me through, pretty much right when I was already by him,” said Ellsbury.

Imagine that: a Red Sox player who puts pressure on the opposition and his own third-base coach.

Red Sox fans have seen their share of tape-measure homers, of sheer displays of strength. They’ve witnessed plenty of fastballs that threaten to register triple digits on the radar gun.

But in their long history, they’ve not often had the privilege of watching blazing speed.

The list is a short one: Tommy Harper, Jerry Remy, Otis Nixon, Johnny Damon, Coco Crisp and, for his short stay here, Adam Stern.

But Ellsbury may be faster than them all. His claim of a 4.2 time in the 40 may be difficult to corroborate or believe, but you don’t need a stopwatch to convince you of his obvious talent.

Enjoy him while you can. Blink, and you’ll miss him.

smcadam@projo.com