Viduka joined Newcastle on a free transfer earlier this summer after seeing out his contract at Middlesbrough.
The Australian international was surprised Middlesbrough did not try harder to keep hold of him.
"To be honest at the time I was surprised. Most people were, including other players, that there was no offer made earlier," Viduka told the Sunday Mirror.
"It is disappointing when you have played for a club and you want to do well and it comes to a point where you don't know if you are staying or going with six months of your contract left.
"You are free at the end of the contract and you have to start thinking of your future.
"Middlesbrough do not look after you in that sense. They never got around to it. The club gets a lot of situations like that. It happened with Bolo Zenden and then me - and it could be Mark Schwarzer below next."
Viduka also shrugged off claims from Middlesbrough chief executive Keith Lamb that the former Leeds striker was only prolific in the final six months of the three year contract.
"He had a lot of pressure on him and he wanted to react. I can't comment on what Keith Lamb says," added Viduka.
"I gave everything in every game. Some games I didn't play well, some I did. If I didn't play well I wouldn't have had any clubs after me.
"I would like to think I played well for Boro. Some footballers go unappreciated but that is a different story."