The Dutch international is on loan at Ajax from English Championship side Newcastle as he attempts to get himself up to fitness following a cruciate knee injury.
The 28-year-old has yet to make a first-team appearance for Ajax with Andre Onana the current first choice, and Bosz says Krul was too optimistic about getting into the side.
He told De Telegraaf: “Tim is a great guy to work with but he’s an optimist. When he came to us, we had the idea that he would work with us in full within two weeks.
“I like his enthusiasm but he wanted to participate with the first team before he had joined any group training.
“We have to be realistic and that would have been dangerous. What he wanted to do we just couldn’t do.”
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.