Conventional wisdom holds that a playoff series doesn’t really begin until the road team wins. That hasn’t happened yet, but already this feels close to done. The Hawks, who won Game 2 on Tuesday 89-72, are too big and too good for these Celtics, who are losing men and luster by the minute.

“That was one team playing at an elite level,” Celtics coach Brad Stevens said, “and one team not.”

The greatest Celtic of all used to throw up before big games. If Bill Russell was watching this first quarter, he might have felt a similar urge to purge. Credit the Hawks for some inspired guarding, but no NBA team — not even the 76ers, who gave up trying a while ago — should look as bad as Boston did over these 12 minutes.

Indeed, no NBA playoff team ever had. The Celtics’ seven first-quarter points was an all-time low in the shot-clock era, which commenced when Russell was a San Francisco undergrad. It also was Boston’s postseason nadir for any quarter. Where have you gone, Hondo Havlicek?

The visitors did open the game understaffed. Guard Avery Bradley was lost to a Game 1 hamstring injury, and 90 minutes before tipoff Stevens announced that backup big man Kelly Olynyk likewise was a no-go.

The rest of the roster must have required a sedative to deal with the news, given the addled way the Celtics sought to attack the Hawks. Late in the quarter, Jared Sullinger looked first to teammates and then to the bench with his arms raised, as if to ask, “What are we doing?” No answer was forthcoming.

The home side dashed to leads of 6-0, 13-2 and 24-3. By then Stevens had burned two timeouts. Nothing changed. His team kept hoisting awful shots and driving into the path of bigger defenders — the Hawks would finish with 15 blocks — and a massive blowout seemed at hand.

Then something did change: Mike Budenholzer started substituting. I know he has his rotation, but isn’t there such a thing as a feel for what’s happening? Would it have been so wrong to keep Kyle Korver, who made four treys in the first 6 1/2 minutes, in a bit longer? The Hawks closed the first quarter using four subs over the final 3 1/2 minutes, in which time they scored not a point.

Actually, the Hawks went pointless the final 5:27 of the first quarter. The Celtics had set an NBA record for futility, but were within 17 after 12 minutes. (They should have been down 25.) By halftime, they’d drawn within 43-28.

By then, neither side was doing a thing, scoring-wise. Both teams closed the second quarter 0-for-their-last-seven possessions. Surely Bill Russell wouldn’t have been alone in his queasiness. This was hard to watch.

The Celtics had drawn within 10 in the second quarter and cut the lead to 11 in third. Unlike Game 1, it never felt as they had a chance to win. They simply couldn’t score enough — Isaiah Thomas, their best player, missed 11 of his first 12 shots; Marcus Smart, who started in Bradley’s stead, missed his first eight — and the latter’s eighth miss was brutal.

With the Celtics down 12 inside the final 10 seconds of the third quarter, Smart cast an off-balance 3-pointer while seeking to draw a foul. He failed on both counts. (The refs called nothing either way this night.) The Hawks rebounded. Al Horford made a trey at the buzzer. The lead was 15 with 12 minutes to play. Given that the Celtics were averaging only 15.3 points per quarter, the game was over.

The Hawks hadn’t exactly dazzled — they made only 34.6 percent of their shots over the first 36 minutes — but their defense was so good (and Boston’s offense so inept) that it hadn’t mattered. They lead the series 2-nil, and the doings of Game 2 suggest that the Celtics as constituted mightn’t be able to steal even one back in Boston. This should be over soon.