Nigel Pearson has warned his side are in for a long and difficult season if they fail to drastically improve at the back.
It was a case of De-Ja-Vu from last season as the Robins scored twice but still left Ashton Gate empty-handed as Sunderland's two strikers Ellis Simms and Ross Stewart had a field day against City's fragile backline.
The visitors took an early lead in the fourth minute when Kal Naismith was dispossessed deep in his own half by Ross Stewart. Simms picked up the ball and unleashed an effort from distance that caught Dan Bentley off guard.
In a topsy-turvy contest, Andi Weimann made it two in two just six minutes later when he stayed composed in the area after picking up Zak Vyner's pass to fire beyond Anthony Patterson.
It quickly became evident that this was not going to be a contest where defences would come out on top but it was the Robins who edged ahead in the second half when Chris Martin atoned for an earlier miss to bundle in Weimann's cross in the 51st minute.
That lead would last just two minutes when Simms fired in his second by getting the better of Vyner before Pearson made two changes by bringing on Kane Wilson and Timm Klose for Mark Sykes and Vyner. Although it couldn't prevent Sunderland from grabbing the winner when Alex Pritchard's in-swinging cross was headed in by Stewart to leave City searching for their first points.
"We're two games in and we've conceded five goals," Pearson said after the game. "I can argue last week that we were unlucky but I can't argue with that this week.
"The first is an individual mistake, the second we've got a throw-in and they score a goal from our throw-in, some really poor defensive work and the third one... it's just too easy for them.
"Even when they scored the third we've still had chances and opportunities to get back in so it's a frustration but two games in and concede five goals. We're giving ourselves far too much to do in games.
"Credit to them they put us under pressure. Their front two were quite lively but we still made basic errors which we've been punished for and that is a big frustration.
"We've dominated possession today and we're not really a possession-based side. But we've had around 60 per cent possession but not been able to do enough with it.
"I still think we created enough opportunities to get something out of the game but unfortunately we've scored two goals at home and still haven't got anything and the big thing for us is our defensive frailty and that's the problem.
"I'm pleased we had a decent pre-season but I don't spend a lot of time dwelling on that. I know that we have a level of physical fitness which is good but unfortunately, we've let two games pass us by when we should be sitting in a better position. It's not a day where I make excuses or look for positives that aren't there. If we concede goals like we've done today, we're going to have a tough season.
"I don't think I need to pick the team up right now quite frankly. They have to feel the pain of losing first and I'll pick them up later, but not now. It's got to hurt them."
City's preparations were hampered by an injury to Matty James in the warm-up, meaning Han-Noah Massengo was handed his first appearance of the season. Pearson admitted his side missed the midfielder's quality in the middle of the park.
He added: "He's got a bit of a calf problem but hopefully it's not too bad. But that doesn't help us. I think we missed his quality on the ball today. It's hard to argue that when the goals that we've conceded are just poor defensive goals."
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