Karlsson was the best player in the first 4 games of that Blues series. Over the first 4 games, the Sharks scored 4 5-on-5 goals with Karlsson on the ice (2 of them were Karlsson's) and allowed 2. Yes, Karlsson scored as many goals as the entire Blues team did with him on the ice in over 80 minutes. Without him, they scored 5 and allowed 9. He assisted on every PP and 6-on-5 goal we scored and we did not get scored on with him on the PK.
I'm not generally one to blame injuries or puck luck for losses but the fact is that Karlsson's injury during game 4 in St. Louis was an extremely fortunate break for that Blues team and probably the biggest reason they won that series. At the time, the Sharks were up 2-1 in the series and down 2-1 in the game in a series that was dead even in goals. The Sharks were pressing extremely hard and had just scored on the PP with an assist from Karlsson. As I mentioned above, he was the best player in that series up to that point. I think they would have tied that game, lost game 5 at home, and then won the series in 6. But once Karlsson got hurt on that back-check and was relegated to bench door opener, it completely deflated the rest of the team emotionally and also pretty much ruined their ability to evade St. Louis' forecheck and make breakout passes.
I still hold DeBoer and a few other people accountable for their pathetic performance in that Blues series. The Sharks were playing a terrible brand of hockey throughout the entire playoffs and being carried by star players in almost all of their wins, and I largely place that on the coach. But I also do believe the Sharks would have overcome that and found a way to win the series just as they did against Vegas and Colorado if not for the Karlsson injury.
People are talking about Karlsson's injury as a "red flag". You know what the real red flag was? The way that the Sharks played in those final 2 games when he went down. They looked like the good guys in a hockey movie before they get good at hockey and the Blues forecheck looked like the bad guys.
Also, nobody should be calling Karlsson soft or questioning his commitment. He put everything out there for these playoffs and should be treated with respect regardless of his decision. I'm sure he saw the articles out there and he knew that playing through injury in these playoffs could potentially hurt his stock in unrestricted free agency but he played through so much pain and found a way to be a positive contributor up until game 5 when he truly was done but still gave it a go anyways. He could have absolutely decided to undergo a surgical procedure to end his season around February when he got re-injured against Boston and that wouldn't have hurt his market value nearly as much as playing in the playoffs did but he never once considered that. He made it very clear as soon as he got hurt that he was going to give it whatever he had in order to try and win a Stanley Cup and he did just that. There are valid criticisms of his performance and his injuries but his heart, grit, and dedication to the game is absolutely not one of them.