Yelich's problem is more likely mental. I sit behind the Brewers dugout at home games so I have a vantage point of being able to see his swing. I also sit behind home plate on other occasions. By no means, am I an expert on his swing but I have watched Yelich's swing closely as my son is learning to bat left-handed. (I also have him watch other left-handed batters too.) One of the things I have noticed is the path of his swing has changed over the course of the last few years. He's coming over the top a lot more than he used to. It seems like he is trying to crush the ball all the time instead of letting his natural swing take over, which is what you see in 2018, 2019. I have no clue why it's changed., It doesn't look like the knee is an issue and perhaps the back is related to him overcompensating for the loss of the natural swing. (Not the back being the cause of the lack of power or whatever, but perhaps the swing caused the back issues? EMT here.) He's pushing it. His swing from 2018-19 is easy, almost too easy, doesn't seem to be trying too hard.