Donovan later set up Hector Jimenez in the 80th minute to make it 3-1, and then walked one in alone in the 87th minute against the helpless MacMath for the final score of the game for the Galaxy (5-3-2).
"I don't think our fitness is a problem," Union manager John Hackworth said. "I certainly think there were some lapses there, and I think we made some glaring errors defensively as a group and we have to correct those. I'm going to go back and look at the tape and try to evaluate all of that."
In the first half, the Union (4-4-3) thoroughly entertained the crowd of 17,031 by pressing the tempo. They were helped by Brazilian midfielder Kleberson, who was making his first start and had quite a few chances early.
After a fluky own goal in the third minute when Donovan's corner kick went off the shoulder and face of MacMath and into the net, the Union controlled play. They got the equalizer in the 23d minute, when Amobi Okugo slid on his knees to head in the ball at close range on a long throw-in by Sheanon Williams.
"Honestly, we played probably the best 40 minutes we've played in that first half, at least 40 minutes," Hackworth said. "We had the two-time defending champions on the ropes in a big way and you've got to be able to sustain that."
Kleberson, who is on loan from Esporte Clube Bahia, said he had been observing the American game in his time on the bench and was ready when he received the call to start.
"It felt very natural," said Kleberson, speaking through an interpreter. "I was content to start this game, and an important game at that. I tried to give my best to the team and I laid it on the field. The Galaxy have a lot of really quality players and that made it difficult."
The flow of the game turned in the Galaxy's favor in the second half. The Union did not put a single shot on net of the three they attempted, and Keane's go-ahead goal shifted the momentum in the visitors' favor for good.
"I'm not sure why that happened," MacMath said. "It was obviously big, giving up a second goal like that. But we lost our tempo and they just kind of took over, and we couldn't get it back."
Cup dates. U.S. Open Cup pairings were announced Wednesday. The Union will face either the USL Pittsburgh Riverhounds on the road or the Ocean City (N.J.) Nor'Easters at PPL Park on May 28.
Contact Joe Juliano at firstname.lastname@example.org. Follow on Twitter @JoeJulesInq