You play either Yu'lon or Chi-Ji.
The choice is between a 2-minute cooldown with a 25-second duration or a 1-minute cooldown with a 12-second duration.
You can pick the 1-minute cooldown if it fits the boss damage pattern well.
The meta choice is the 2-minute Yu'lon.
Chi-Ji buffs fistweaving and provides extra smart-heal throughput for 12 to 25 seconds. It fits well in fights with constant damage, as long as the monk has melee uptime.
Yu'lon is more complex. It consists of two phases: the ramp and the dump. During the ramp, the monk spreads out Enveloping Mists, requiring them to plant their feet and cast. Afterward, they need to stay in place again to spam Vivify.
Because of this, when assigning Yu'lon, the monk cannot be expected to move, as any movement significantly reduces HPS. This can be mitigated by giving them Spatial Paradox from an Evoker.

Assign Chi-Ji when the monk has a window to output extra healing and has uptime on the boss.
Unlike Yu'lon, movement does not negatively impact this cooldown as long as they can attack the boss.