The Tigers had hoped to make Long their record signing on Monday, with a £5.5million deal agreed in principle.
"It's enormously disappointing, of course, but West Brom had always made it clear it would be conditional on them doing their own business," he told the Hull Daily Mail.
"They came to the stage late in the day that they no longer wanted to do the deal.
"That's understandable because you look after your best interests as a football club, but it was hard to take coming so late in the day. Everyone I have spoken with since has said 'that's football' but it's still hugely disappointing."
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