There is no doubt this hitter is a premier player but the ability to stay healthy is a major gamble.
When Michael Brantley can play a full season he puts up great numbers. A career .297/.354/.440 slash will back that up. Keeping him on the field is part of the risk you must be willing to take.
The Astros put Brantley at designated hitter more times than in leftfield hoping to preserve the bat in the lineup. He will turn 34 next May and despite projections that he could fetch a 3-year deal at roughly $21 million per, that seems pretty overpriced. Teams may make the DH move permanent to ease the wear and tear.
The issue for the KC Royals would be that as of now they have a steady DH in Jorge Soler and really could use the outfield help, especially on defense with the retirement of eight-time Gold Glove winner Alex Gordon. Signing Brantley puts too much hope in that he would make it through the season without spending possibly a considerable amount of time on the injured list.
One much cheaper, and less glamorous possibility is Josh Reddick. We are a long way from the 2017 season where he hit over .300 while chipping in 51 extra-base hits.
Reddick seems pretty durable sticking to the outfield for the majority of his games but he also is less than stellar with his glove. Overall he had a negative WAR for 2020 but if he could put up numbers similar to last year with 15 homers and a .275 average it would sadly be a big improvement for a Royals corner OF.
I think a 2-year commitment for $3-4 million is not unreasonable and would keep the seat temporarily warm while we see what all outfielders in the organization are ready to contribute at the major league level.
Houston does have a catcher whose contract has expired in Dustin Garneau but he would not add anything to the KC Royals that they do not already have under contract.