The 28-year-old Holland international joined the Eredivisie outfit on loan for the season in August following his return to training from cruciate ligament damage which had sidelined him since October 2015.
However, despite being signed as a replacement for Barcelona-bound Jasper Cillessen, he is still to make his Ajax debut with deputy Andre Onana having made the most of his chance.
Bosz 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
[The situation with Onana] was obviously intended to bridge the short period between the departure of Jasper Cillessen and Tim being fit. But Tim was not fit and Andre has done well and is reliable.
"All of that comes into consideration for the choice for the rest of the season. On the one hand it's a luxury for us, but naturally for Tim it's a nasty situation