The Black Cats scored a goal in each half via Jobe Bellingham and Wilson Isidor, and were the dominant side for the vast majority of the game.
Sunderland stroked the ball across the Stadium of Light pitch with authority and opened up the U’s defence on a handful of occasions.
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Brown said: “It was very tough. They’re a very good team and they are where they are for a reason, but I do think with the levels we can reach, we were no way near it.
“They capitalised on it, and it was a poor performance from us. We’ve had a good start to the season so far, and I think it’s probably the first game where we walk away, we’ve lost and we’re disappointed.
“The result was disappointing. We never want to go anywhere and lose, but I think it’s more the performance.
“I don’t think we were at the levels we should’ve been, and I just think we were off it for whatever reason that is.
“It’s probably the first time in a long time that’s happened. It’s just about bouncing back.
“You’re always going to get everything from every player, all their energy, the attitude, but I just don’t think it clicked.
“It just didn’t work in the ways it has worked this season so far. There’s got to be a little bit of credit to them, they’re a very good team and made it difficult for us, but I don’t think we helped ourselves at times.
“We’re here, and we’re here for a reason because we did well last season. The way we started, we’re probably looking at some games where we expect a lot more points than we’ve actually got.
“I don’t think we’ll ever look at this as coming here as a test, it was coming here for three points.
“Obviously we didn’t get that, and we didn’t really get the performance either, so we’re very disappointed in the outcome overall.”