Any chance Fury underestimated Usyk?

No I don't think so, he clearly trained very hard to get himself back into shape after showing up looking like a muppet vs Ngannou.

But he did get off too a slow start, he gave away too many of the early rounds and than cost him the fight. Usyk is known to be a slow starter but he allowed Usyk to dictate those early rounds and we all know Usyk comes on strong late in fights so to do that he dug himself a hole.

I think he was surprised at how easy it was for Usyk to get into a position too land and eventually adapted by upping his work rate in those middle rounds to keep Usyk from simply continually pressing. But he clearly wasn't prepared to fight at that pace for 12 rounds and even if he was Usyk was always going to ramp it up later in the fight like he usually does.

In the end Fury's lack of professionalism during his entire career cost him. He's get notoriously out of shape at times and abused his body with drink, food and drugs over many years, While Usyk has been nothing but the consummate professional his entire career, focused, dedicated and disciplined, made those sacrifices over many years and at this level that can be the difference between a win and a loss.