On any given night Rielly can make a play that drives me up the wall. That same player on any given night can make a play that pulls me out of my seat screaming, yaaaaaa! How do you evaluate that type of player? You evaluate him on his weaknesses and strengths not on any given night, but on his cumulative effects over a sustained period of time. Morgan Rielly is not a perfect defenseman, but the Maple Leafs benefit from having him on the team as his ability to drive play exceeds his occasional brain farts. Simply put, we score more goals with him on the ice than we have scored against us when he is on the ice. You win games with those types of players, ya can look it up.
Another aspect to consider are his contributions to the team as person and leader within the locker room. I've never heard or read a word said against the kid, from any player, past or present. What I have heard and read are countless compliments directed towards a person who is respected and appreciated. Players like playing with guys who they respect and appreciate, ya can look it up.
There is a certain segment in the fanbase of the Leafs that have a need to belittle and criticize because they focus on the imperfections of a player and not the overall picture, ya can look it up.
It's a flat cap world, make Morgan your top paid D at 6M/ for 5 years, a 1M/yr raise, front load it to keep the real dollar value low so that if there is a need in the future to move on, it's an easier deal to construct.