Maybe he was underrated as a player but I am so tired of people acting like Lendl is the key ingredient to Murray's success. It's like claiming that Sampras and Fed would be nowhere without Annagoat.
Here's my two cents:
1) Lendl is extremely taciturn and phlegmatic. This MAY temper Murray's outbursts but it also means he is doing nothing to motivate Murray. Murray has enough cheerleaders anyway.
2) Has Lendl really made any obvious technical changes to Murray's game? I have not really seen anything obvious.
3) Coaches in general are highly overrated in general because half the commentators are coaches themselves and naturally want to promote themselves.
I think you will find that Lendl has helped (persuaded) Murray to make some key changes in his game to help him become a champion. I will explain where:
Murray had many issues with his forehand. When Murray went crosscourt, due to the grip he was using, was not penetrating the court, his ball often dropped short which made him vulnerable. Murray was also often pushing the ball down the middle of the court which surely you must be aware of. What Lendl persuaded Murray to do is hit through his forehand much more, do not pull up on the shot which drops the ball short, hit deeper and into the corners, in other words take a few more risks, of course, when you hit the ball in the corners, you are more likely to make mistakes. At the highest level it is about risk and reward, the talented ones who are prepared to take the risks, are more often likely to get the reward.
It appears that Lendl has also persuaded Murray to stand closer in to return serve, standing too far back opens up the angles on him, especially by Federer who can exploit that.
Those are the technical adjustments Murray has made under Lendl. Murray was also improving his 2nd serve but Lendl would have reminded Murray to keep moving the ball around the box at least 90mph and deep more often than not. Until recently, Murray was never brave enough to hit 2nd serves to the forehand on the ad court. Players take a longer swing on the forehand, that is why you will find the likes of Sampras regularly served there on the 2nd serve, not just a regulation 2nd serve to the backhand which can often get hammered.
Now, you will also find that before Djokovic hired Boris Becker, he had exactly the same problem with the forehand. Djokovic was often pulling up on the shot and was unable to finish points. Becker changed all that, Djokovic's forehand now has much more penetration and he hits much better in the corners too. Remember, Djokovic lost a handful of major finals before he hired Becker.
I recommend you watch a re-run of the 2013 Wimbledon final. I think you would be shocked at how difficult it was for either player to hit a winner, both were pulling up on their forehand time and again, hence a 3 set match took almost 3 hours and 30 minutes.
