The A’s, the defending American League West champions, beat the Yankees, 5-2, on Wednesday night and improved to 40-27, the best start for this franchise since 1990, in the powerhouse days of Mark McGwire, Rickey Henderson, Dave Stewart, Dennis Eckersley and the American League pennant.

In terms of star power, this team cannot stack up to that one, but they have certainly looked better than the Yankees. They moved into first place in the A.L. West and have the second-best record in the American League.

They were led by Brandon Moss, who hit two home runs. That’s more than the Yankees have hit in almost a week.

The Yankees haven’t hit a home run in five games and managed to score only 12 runs in that power-starved span.

Gone are the days when some people would actually complain because the Yankees relied too heavily on home runs. Now there is a perplexing shortage.

“Yeah, that’s too much,” said Mark Teixeira of the homer drought. “Bronx Bombers.”

Teixeira hit the Yankees’ last home run when he and Robinson Cano went back-to-back in Seattle on the first day of the West Coast trip last week. That was the Yankees’ 70th home run of the season, which placed them ninth in the American League.

Against Bartolo Colon on Tuesday and Dan Straily on Wednesday, they managed six hits and two runs combined.

“You hate to say it again, but we don’t have home runs right now,” Teixeira said. “It’s tough to string together a whole lot of hits against good pitching. We’ve faced some good pitching the last two nights. Bartolo was really good last night and this guy pitched really well tonight. Sometimes you need to pop a couple of home runs off those guys, and we haven’t been able to do it.”

Cano has cooled off from his torrid start, as have Vernon Wells and Travis Hafner, who went a combined 1 for 15 Wednesday. Cano has one home run in his last 40 at-bats.

“We’ll keep working hard and find the answers to get these guys back on track,” the hitting instructor Kevin Long said. “It will happen.”

But even more worrisome is that while the hitting has suddenly fallen of, the pitching has not been as strong as it was earlier. Phil Hughes had another disappointing start in which he could barely command any of his pitches — he walked five batters in only four and one-third innings — and both he and C. C. Sabathia have had trouble stringing together dominant starts in succession.

The Yankees look to Hiroki Kuroda on Thursday to avoid the sweep, but he has not been pitching well. Kuroda is 0-3 with a 5.23 earned run average in his last four starts, all of which the Yankees lost.

Trailing by 3-0 after five innings, the Yankees manufactured a run in the sixth on a sacrifice fly by Teixeira, then scored again in the seventh when Kevin Youkilis was hit by a pitch, advanced on a wild pitch and eventually scored on a single by Jayson Nix.

Joba Chamberlain snuffed out an Oakland threat in the seventh, but the A’s got their insurance run in the eighth on Moss’s leadoff home run, a long drive to straightaway center field.

Hughes gave up three runs and four hits and had control problems. He was coming off one of his better starts of the year; he held the Mariners to one unearned run in seven innings six days earlier, in the game where Cano and Teixeira last homered.

“Every single time I wanted to make a big pitch, I wasn’t able to execute,” he said.

The Yankees had difficulty with Straily, a right-hander whose earned run average was 4.67 coming into the game. He allowed only two base runners thought five innings, one of which was an error, and he set down 11 in a row before Brett Gardner drew a one-out walk in the sixth. Gardner went to third base on Cano’s high-bouncing single into right field, then scored on a sacrifice fly to right by Teixeira. But then Straily struck out Hafner on a full-count slider to end the inning.

The Athletics jumped on Hughes in the second inning, using the long ball to seize the lead. Josh Reddick reached on a single to right, and then Moss, who has surprising power, hit the first pitch from Hughes, a changeup, into the right-field stands for a two-run home run and a 2-0 lead.

Ichiro Suzuki backpedaled and then stopped, looked up and appeared to have a play on the ball. But it was only a ploy to fool the runner, Reddick. Ichiro wanted to prevent Reddick from running all out in case the ball hit the wall. Suzuki stopped and spun around quickly, but the ball was not only over his head, it was over the wall, too.

Moss took Joba Chamberlain deep in the eighth, too, this time on a fastball.

“He hit two balls up,” Manager Joe Girardi said. “he hit a changeup up and he hit a fastball up. We made mistakes and he made us pay.”

That’s the sign of a good team. Perhaps not as good as the 1990 A’s, but apparently better than the Yankees.

INSIDE PITCH

Mariano Rivera, wearing his batting practice uniform, surprised Julie Vasconcellos, the Athletics’ longtime mailroom attendant, with a visit to her work station before Wednesday’s game. Rivera spent about a half-hour with her and then walked through the front office chatting with other employees and posing for photographs, as if in a scene from a “SportsCenter” commercial. The gesture was made as part of his farewell tour across Major League Baseball. Rivera, who announced in spring training that this would be his final season in baseball, has been meeting with team employees and fans in each city he visits for the last time.