I hate to say it but Monahan's a long shot. Not because he hasn't been playing well because he has been, but I'd put a few players ahead of him even without Hertl.
In no particular order:
Seth Jones
Anthony Mantha
Valeri Nichushkin
Martin Jones
Nathan MacKinnon
all have a strong case either alongside or in front of Monahan right now. Monahan is not a bad player by any means but he's not exactly flashy or putting up "wow" numbers in his respective categories like Martin Jones is, for example.
Just a very, very tough Calder year.
All credit to Monahan if he does pull some bigger numbers but I feel like this season a rookie would have to score 55+ points as a forward or have incredible defensive ratings to win. But Monahan has never been that kind of "attention-garnering" player - he plays so steady, composed, mature, and serious and that can work against him especially in regards to awards that don't have as much empirical basis to them. The Calder trophy is a bit of a popularity contest + some stat watching. IMO, Brodin should have won it last year, for instance, but he was neither popular enough or statistically excellent enough.
IMO, Calder should be awarded like the Ted Lindsay - judged by their peers of rookies and sophomores and the like. Never been a fan of the Professional Hockey Writers Association in terms of dishing out awards - as we all know, some media can be pretty clueless about the state of the League as a whole. Especially since Jarome Iginla lost out on a Hart trophy by one vote because some dork didn't even put Iginla's name on his ballot whatsoever, and cemented Jose Theodore as a Hart winner instead.