The discussion was about scouting, not judging performance after a player has been a professional. You can't scout based on stats. High School stats mean nothing. For the most part, college stats mean nothing. Scouts evaluate tools. They judge arm, speed, and power first, because the belief is that everything else can be learned through coaching. Stats don't mean a thing until the minor leagues and even those don't have a complete correlation to success or failure, and by the time a player is compiling meaningful stats, he has already been drafted. Drive and desire are very hard to evaluate, because you need to invest a lot of time observing the player, and scouts cant devote so much time to individual players. It is what makes talent evaluation so imprecise.
As for your example of Manny Ramirez not having heart, you are mistaking hustle for drive and desire. While Manny may have been a jerk who didn't want to be bothered running hard etc., his desire to be a great hitter was unmatched. He never missed his batting practice, and I can tell you that I witnessed him several times take live bp in the outfield cages at City of Palms Park in Fort Myers after taking his reps on the field. He would hit continuously for another 30 minutes stopping only to change pitchers and restock baseballs. It was my favorite part of spring training watching him take his private bp. It was pretty awesome to see. The guy was phenomenal to watch. His swing was so fluid and looked so easy, but up close you could see the power and speed and the last instant adjustments to pitch location. Manny loved to hit and his passion drove him to excel. He took far more private bp than the other players. Also, he played in most spring training games (almost every home game) and he never came out until he got his 3 or 4 ABs.