Filip Hrgovic praised Moses Itauma after his shock ninth-round stoppage but said his opponent had boxed “like an amateur”.
Hrgovic, 34, on Saturday night to win the vacant IBF heavyweight title.
Itauma, 21, who was unbeaten coming into the bout, was dominant in the first six rounds after landing a string of accurate blows.
But he became visibly exhausted as the fight progressed and he failed to find a way past Hrgovic's durable chin, with the seventh round being the first time in his career he had been past six rounds.
The Croatian then pounced in the ninth round, which forced the Brit's corner to throw in the towel.
Following his win, Hrgovic hailed Itauma as the most talented fighter in the heavyweight division but said his boxing reminded him of the amateur ranks.
Hrgovic said: "Let me be honest, he's amazing. I didn't know it was going to be so hard, I thought I was going to handle him easier.
"I was losing all the rounds. The kid is amazing, he was so fast, you don't see his punches coming.
"He doesn't hit so hard but he's fast. But since the first (round) I saw he was gassing out, he was boxing still like an amateur.
"He's inexperienced, I saw he was gassing out and I knew my only chance was to stay there to make him tired, to keep coming and to maybe stop him later on. That's what I did.
"I came to London, his hometown, against the most talented guy in the division and I beat all the odds.
"I want to be considered the best heavyweight in the world, and this is my time. I am the best. I am the present and he is the future.
"I want to fight for unification against Dubois or Agit Kabayel. I want to go for unification, that's what I want."
Itauma came into the fight lauded as one of the sport's brightest young talents after some early knockout wins.
But he will have to work on pacing himself for fights that go beyond the halfway point against durable opponents like Hrgovic.
Itauma was carried from the ring on a stretcher and taken to hospital following the stoppage as a precautionary measure.
On Sunday morning, a statement from Queensberry Promotions confirmed he had since left hospital and was home with his family.
The spokesperson said: "Moses is well. He left the hospital last night and is back at home with his family."
Promoter Frank Warren had earlier said: "That's what we always do to make sure because the most important thing is the boxer's health.
"He's there and he's in the best place. We'll see what happens when I've got a report. We'll obviously let everybody know.
"He's done something to his leg as well and he was quite tired. It was a high-paced, fast fight for both the guys. It was just edge of your seat stuff all the way through."
Itauma came into the fight hoping to be crowned Britain's youngest ever heavyweight world champion, but Warren said it was now unclear what his next move would be.
Warren added: "I don't know what he's going to say. I'm sure he's a fighting man. He showed that right to the end. At the end, that's the type of fight it was - they were standing there just letting bombs go.
"I don't even know how he feels mentally. Mentally, he's going to be completely down. We're going to have to talk to him and lift him and see what he's like.
"It's going to take some time. He's going to have a nice break. It's not even the time to talk about it with him at the moment.
"It's something we're probably doing in two weeks or even a month's time, see where we go with it."
Hrgovic must now take on the IBF's mandatory challenger Frank Sanchez.
