Any team in the NBA is going to be worse if they suffer a major injury to one of their core players, and the Cavaliers are no exception. If any of their stars miss extended time they will play at a worse level, and if any are out for the playoffs their chances of advancing are reduced to near zero.
The player they can least afford to replace, however, is clearly Darius Garland. If Donovan Mitchell misses time the Cavs are a worse offensive team, no question, but they can insert Caris LeVert into the starting lineup and play similar basketball at a lower level. Evan Mobley is a truly special defensive player, so no “replacement” is going to step right in seamlessly, but the Cavs have both another elite rim protector in Jarrett Allen and a ready-made Mobley to plug in in Isaiah. They could make it work, as they similarly could with Allen out.
If Garland were to miss extended time, however, the Cavaliers would be in trouble. Assuming that Sharife Cooper doesn’t make the final roster, the only true point guard to replace Garland is Ricky Rubio, and he is away from the team for an indeterminate amount of time for personal care. Ty Jerome is the nominal backup, but he is a combo guard and not a true floor general. Mitchell and LeVert are both reasonable playmakers on the wing, but they have nowhere near Garland’s level of court vision.
It will help that the Cavs are installing elements of a motion offense to maximize their new shooters and out the ball in Mobley’s hands more often; that would help them survive a long Garland absence. It’s possible that undrafted rookie Craig Porter Jr. would be ready to step up or that Cooper could be called up from the Charge, but neither combined Garland’s passing and shooting. Losing Garland for an extended period would likely derail the Cavs to the point of a failed season.