Summary
The '19-'20 Kings were mostly a disappointment, regressing a bit after surprising the league in the '18-'19 season by winning in 39 games. However, Holmes was a bright spot in that season of disappointment, as he was the one new signing that worked out well. Holmes became the Kings starting center after signing a two-year, $9.8 million contract in 2019, and should be able to turn his play over the last two seasons into a raise this summer.

Holmes finishes really well at the rim. He's athletic enough to be a good lob threat, and he also has really nice touch on his floater. He has nice instincts as a roller, knowing how to time his movements in order to stay in the passing pocket. He also has been an above-average offense rebounder for his position each season of his career, per Cleaning the Glass.

On defense, Holmes blocks shots at a really good rate, but isn't a high-level rim protector. His defensive numbers aren't bad, as opposing teams tend to shoot worse at the rim with him on the court than when he sits, but I'm not sure that's reflective of Holmes's defense. The fact of the matter is that Holmes is the anchor of a defense that was worst in the league this season. While Holmes isn't the sole person to blame, it certainly doesn't reflect well on his ability to protect the rim. Holmes is probably someone who is best served as a high-quality backup center, but has shown he can start for some teams in the league.
 
Cap Considerations
Holmes will be an unrestricted free agent with a $6.5 million cap hold and Early Bird rights. Since the Kings will most likely be operating over the salary cap, they will retain Holmes's Early Bird rights until he is re-signed or signs elsewhere.

The Kings could offer Holmes a starting salary of up to 5% more than the average salary (likely around $10 million) using the Early Bird exception, which may not be enough to re-sign him. If it's not enough, the Kings would have to offload some salary to free up cap space to give Holmes more than the amount allowed under the Early Bird Exception.

If Holmes demands a starting salary of more than the full $9.5 million Non-Taxpayer Mid-Level Exception, only teams with cap space will be able to sign him, but it's possible that Holmes could be had for the full MLE.