Monahan is above average defensively. He has elite positioning, stick work and smarts. There is a reason he is a top 10 forward in the NHL in takeaways since his rookie season and neck and neck with the likes of Barkov, Backlund, and Orielly who are all considered elite two way players. His giveaway/takeaway differentials are elite as well. There is no denying that when healthy over the past two seasons Sean Monahan has taken his game to another level and been an absolute beast, but unfortunately a string of injuries has derailed his ability to perform from start to finish in both seasons.
Does he have a few downfalls that can in certain situations limit his abilities? Of course. His first few steps are below average, and as others continuously like to point out he lacks that physicality that you would hope for from a player his size. In certain mathchups he can be exposed due to those shortcomings. He's not perfect but he is an incredibly talented player that get's unfairly ripped on by this fanbase.
Calgary is incredibly soft down the middle of the ice as a whole. There is no denying that, and Monahan is not the only player contributing to that. We are in tough to match up against any big, fast, strong team as we are currently constructed. You can absolutely win a cup with Monahan as your "#1 C" but he needs a more suitable 2c behind him as Backlund, Ryan and Jankowski also all lack that element to there games as well. It is no coincidence that Treliving targeted Kadri to add some snarl and bite to the center position.