I don't think that Monk can work as a main healer. As a secondary healer yes, but it can't substitute a Druid because the Druid's sio is a one second cooldown spell, while Monk's sio is a two second cooldown spell. It might work on some respawns like Megadragons, but not on hard hunts like Libraries where if the Knight stays one turn without healing, he has a high chance to die. Unfortunately, even buffing his Magic Level and healing capabilities, he probably won't hold the healing like a Druid does.
Cipsoft already stated that both Knight and Druid main roles are Tanker and Healer, so a Monk can't take the role of a Tanker and neither the role of a healer. If he tries to tank and get full box on a party hunt, he loses his serene status which debuffs him so much that his damage and healing suffers a lot, he also doesn't have a spell like Challenge (Exeta Res) to get aggro from monsters like a Knight can do. And if he tries to be the main healer, he will still be behind Druid's sio with a one second cooldown difference.