Here's my train of thought on the subject. It's possible to play on torn ACL in certain circumstances, especially with world-class athletes. If your leg muscles are strong enough, you can get by without the ACL. Granted, the injury is rather painful and unpleasant. Here are the two problems in particular I had with Yost's move: 1) The ACL is usually injured in bad landings. They can be injured when a person's leg is unexpectedly and sharply twisted or turned in a certain direction. However, they can also happen when someone lands flat on their heels due to the pressure the structure of your legs exert on the knee. However, they are difficult to initially diagnose because a lot of times, what appears to be an ACL tear could in fact be a hyperextended knee, which is a less serious injury than an ACL tear. Surgery might not be required. Rest and rehabilitation could get the knee back to full strength over the course of a few weeks, depending on the severity of the injury. Recovery would be nowhere near the amount needed for a torn ACL. In Gallardo's case, it very well could have been possible that he had hyperextended his knee or maybe just partially torn his ACL. On the spot-testing can be inconclusive in those instances; you need MRIs in most cases in order to be sure the ACL is completely torn. Moreover, pitching involves a lot of hard landings and sharp turns for the legs due to pitching mechanics and the need to field nearby groundballs. Also, pitching mounds usually do not offer the best traction in the world, meaning he was at risk for a slip and fall. If he could have had a partial tear or just a hyperextension without much damage, why risk completely tearing Gallardo's ACL? 2) The other big problem I had was, as I mentioned, these injuries can be rather unpleasant and painful. I don't know if it was his plant leg or his landing leg that got injured, but those two mechanical aspects of pitching are important. If you incorrectly plant your foot or loosen up on your follow-through for your landing foot, you'll be putting a lot of pressure on other parts of your body. Your legs do a lot of important work in pitching. If your motion gets screwed up or if you have to compensate for a loss of velocity by getting other parts of your body involved, you're risking further injury. Quite frankly, it was stunning how dumb Yost was for trotting Gallardo back out onto the mound. Welcome aboard, by the way.