Leeds were good value for the points as United saw defender Lisandro Martinez for pulling the hair of striker Dominic Calvert-Lewin. The red card came after Leeds took a 2-0 first-half lead through a Noah Okafor brace.
"They'll be disappointed," Keane said on Sky Sports. "They've had a lot of momentum over the last few months, a lot of praise for the manager.
"There's obviously been concerns going into the game that they haven't played for a number of weeks. But the manager, Carrick, said that shouldn't be a problem, they've had a training camp.
"But it was the way that they started the game - you start the game that slowly and it's hard to recover. And they didn't in the first half. And what you've got to do in this game - we saw Man City do it in the second half yesterday - you've got to run!
"They weren't sprinting, United. And the bit at the end was too little too late. It's a big setback for them. Midfield? Non-existent, the two of them, particularly in the first half, no energy, no quality. Leeds were out of the blocks quickly and it was a fantastic result for Leeds."