If he were to play these two simultaneously, who would he play when both of his bigs are resting? Viacheslav Kravtsov, I guess, is not ready for real minutes.  So, Frank plays both of his best players at different times at the same position. 

Perhaps the real reasons that Drummond is not playing with Monroe is because Viacheslav is not ready to play and Maxiel is serviceable.   

Has anybody considered the idea that not playing the two together may actually help Drummond’s development?  If Monroe’s skill set covers up Drummond’s deficiencies, then Drummond may not be forced to develop those much needed skills.  It is one thing to try them in practice, but another to apply them in games.  Frank seems quite intentional with his moves, so perhaps he is intentionally putting Drummond in this position to force hime to develop different parts of his game in game situations.  Monroe’s presence on the court may adversely affect the situations that Drummond is forced into.