The Montreal Canadiens completely revamped their left defence in the summer of 2017. They traded Mikhail Sergachev to the Tampa Bay Lightning, dealt Nathan Beaulieu to the Buffalo Sabres, lost Alexei Emelin to the Vegas Golden Knights in the expansion draft and allowed Andrei Markov to walk away via free agency.
That is three proven NHL caliber left defenders leaving town in the same offseason. Also heading out of Montreal was one of the top defensive prospects in the game. That is a lot of turnover for one summer.
When you move on from every one of the defensemen that played on the left side in one offseason, you have some major holes to fill.