The U’s were dominated by a Black Cats side which stroked the ball across the Stadium of Light pitch with purpose, and won possession back remarkably quickly.
Sunderland scored a goal in each half via Jobe Bellingham and Wilson Isidor, but could’ve added more if not for a handful of stops by Jamie Cumming.
United head coach Buckingham said: “That’s a top team that are in top form at the moment, at home and showcasing what the level of this competition is.
“We can try and set ourselves up to come here and do what we can, and we’ll continue to do that, but when you come up against teams like that, that can happen.
“It’s very different to being at home at the Kassam, and it’s not one we accept, of course.
“It’s one we’ll continue to delve in and try to get better with, but that can be the reality of this league.”
Buckingham also spoke about the difference between playing at home and away in the Championship, pointing to a conversation he had with Derby County head coach Paul Warne in midweek.
“We’ve welcomed West Brom and Burnley, and they’re very similar to what Sunderland will be at the end of the year,” said Buckingham.
“I’ve been speaking to a couple of the managers afterwards, and being at home does make a big difference.
“I spoke to Paul Warne this week, and he said going away feels like you’re in not just a different league, but a different country sometimes.
“You saw that. They’ve come off the back of two very good results, there were 40,000 people here at home, feeling confident, and they’re at the top of the table for a reason.
“But we need to make sure we do everything we can to get ourselves into a game, and we tried to do that.”
On the goals his side conceded, Buckingham said: “The first one they had some rotation down their right-hand side, down our left which we didn’t manage, and then the cross into the box caught us.
“And then the second one, we got caught trying to play out, and even then, you can’t really take too much away from the strike.
“We left ourselves exposed, but I think most of the time in League One and the Championship, to get that kind of finish running onto a ball being played over your shoulder, and finish the way he did, it’s a wonderful finish.
“It’s either that or we go back to playing route one trying to just get ourselves up the pitch. It’s striking a fine balance between having an identity about who we are and maybe not putting too much risk with the ball at times.”