Summary
Mykhailiuk's value is pretty clear, and that's as a shooter. The former Jayhawk has shown legitimate ability to hit tough 3s. He can hit shots coming off pindowns, and can shoot over tough contests by defenders. At 6'8", he has legitimate size, similar to what makes Duncan Robinson valuable (beyond his other-worldly shooting ability). However, his career 3-point percentage is only 37%, and he has only hit 35% over the past four seasons.

Despite the Raptors not having a ton of shooting, Mykhailiuk was unable to get consistent minutes for Toronto in '21-'22, and he didn't get any non-garbage time minutes for the Knicks in '22-'23, per Cleaning the Glass. While he did play some after being traded to Charlotte, the Hornets' season was already down the drain by the time he arrived. He didn't get many minutes for the Celtics in '23-'24 either.

Mykhailiuk doesn't have much skill off the bounce, and is generally a poor defender. Along with hitting shots at a higher level, developing those other areas of his game would be huge in getting him more minutes.

Despite playing four years at Kansas, Mykhailiuk entered college really young, so he's still only 26 years old. While that's not super young, it's still young enough to expect some growth, even if the likelihood of vast improvements is pretty low.
 
Cap Considerations
Mykhailiuk will be an unrestricted free agent with a $2.1 million cap hold and Non-Bird Rights. This means that the highest starting salary the Celtics could offer him using the Non-Bird Exception is $3.2 million. This should be enough to retain Mykhailiuk, since he settled for the minimum of $2.3 million to come to Boston. Alternatively, the Celtics could use their Taxpayer Mid-Level Exception (if available) to re-sign Mykhailiuk if he demands more than $3.2 million. 

Other teams will have multiple ways to sign Mykhailiuk as even teams without cap space could possibly use any of the Non-Taxpayer MLE, Room MLE, Taxpayer MLE, or Bi-Annual Exception to sign him. It's also possible that Mykhailiuk only gets minimum offers again, and may not even get fully-guaranteed offers.