He won’t be too hard to replace. Put it that way.
He certainly isn't tough to replace on the Canucks, not having played for them nor being anywhere close to having earned a callup.
Otoh, as of right now the Comets have two centres (real centres as opposed to wingers who still show center in their listing) those being Kero and Darcy. Cassels would not be a veteran under AHL rules so he wouldn't limit what other skaters can dress in Utica.
He's replaceable in Utica, but he wouldn't have been without value there.
I have no concerns about not having him in the Vancouver prospect pipeline, but do wonder if the Comets will actually take some action to replace him.
Madden is a much better prospect
This will be a popular impression but it gave me a couple of sober thoughts.
Cassels was a 3rd round draft pick in 2013 after scoring 43 pts in 64 games and going +7 for the Oshawa Generals of the OHL.
Madden was a 3rd round draft pick in 2018 after scoring 34 pts in 50 games and going -5 in the USHL. He's also listed on hockeydb as weighing 152 lb.
When drafted Cassels was a little less than average age to be draft-eligible for the first time. Madden is older than the average 1st time draft eligible prospect.
Madden is young so he has more potential-but he most certainly isn't at this stage such a good prospect that we can count on him for the future, nor be happy getting rid of a prospect because we've got Madden in the pipeline. Nor is it clear that he is better as a prospect than Cassels was five years ago and he probably isn't as good a prospect as Cassels appeared to be three years ago.