Despite going down 3-1 to Aston Villa on Saturday, Hull's dream season continued as they were guaranteed a Europa League berth thanks to Manchester City's win over Everton.
It meant their FA Cup final opponents Arsenal were guaranteed a UEFA Champions League place and confirmed Hull will feature in Europe's secondary club competition.
Norwich's draw at Chelsea a day later mathematically confirmed Hull's top-flight survival and Bruce described the achievement as "remarkable" but stated they cannot afford a similar performance to that at Villa Park in future.
"On an awful day, if someone had said to me two years ago we'd be safe in the Premier League, planning for a FA Cup final and in Europe, they'd be carting me off in white coats," he told The Hull Daily Mail.
"It's quite remarkable. But we'll not last long in Europe playing like that.
"We won't play that badly again. We couldn't be any worse."
Ashley Westwood's opener and an Andreas Weimann brace helped down Hull at the weekend as their mixed recent form continued.
Bruce's men have taken just seven points from their last nine league games and Bruce claims the Villa defeat should serve as a reminder to maintain standards.
"It might be just a gentle reminder. If we were getting ahead of ourselves, football has a habit of bringing you down a peg or two," he added.
"I haven't had to rant and rave on many occasions but that was so not like us.
"We were lackadaisical, we didn't win anything at all that we shouldn't have done.
"That upset me more than anything. It's not like us. We've prided ourselves on being tough to beat and tough to break down."
