READ MORE: Irate Hull FC coach Lee Radford slams his players who “turned into refs”
They looked capable of making it three wins from their opening three games of the season when leading 6-2 against champions St Helens on Sunday.
However, a disastrous start to the second period - when woeful Hull conceded four tries back-to-back - left them trailing 24-6 by the 56th minute and out of the game.
“We were pleased with the first 40 and then obviously disappointed with that second half,” said Shaul.
“We spoke all week about how it would need an 80-minute performance and to not do that is disappointing.
“If we want to be up there challenging in Grand Finals we need to be better than that.”
The Black and Whites lost right winger Ratu Naulago to a hamstring at half-time and, with centre Mahe Fonua moving out wide - and crucially second-row Josh Griffin moving to left centre - Saints ruthlessly capitalised as they targeted the home team’s rejigged right side.
“It’s never good losing players through the game and obviously we lost Josh Bowden in warm-up as well but that’s no excuse,” added Shaul.
“Our edge there has got some work to do, me included.”
Coach Lee Radford bemoaned some of his side for “turning into refs rather than rugby players” as they contested a number of decisions they deemed went against them.
Shaul - who scored a consolation try with the final play of the game - insisted: “I didn’t really notice to be fair. I’m not one to get at the ref so that’s up to those players.”
They head to Wigan Warriors on Sunday and he maintains they can bounce back.
“Yes definitely and that’s the best thing about sport; it’s not long til you get to go again and right those wrongs,” added the long-serving 27-year-old.
“We’ll all be hurting and it’s a tough one to take but it’s gone.
“It now has to go to the back of our minds as we all look forward to Wigan and trying to get the buzz for that.
“We can’t sweep it aside. We’ll review and learn what we’ve done wrong and hopefully make those things right.
“But we can’t dwell on it; games come around too quick.”