There are 4 ligaments in your knee not 3 - ACL, PCL, MCL, LCL. You certainly cannot "feel" if you ACL is torn by extending your knee. Below your knee cap is your patellar tendon which tightens. This is totally different then your ACL which is deep in your knee, and it is actually common practice to take a 1/3 of your patellar tendon and use this as a graft to replace the ACL. The two most common mechanisms to cause an ACL tear are a blow from the side which causes a shearing force of the ACL onto the PCL and a hyperextension of the knee which shears the ACL off when it contacts the femur. It looked like this was more of the hyperextension type injury from what I have seen. Probably the object most at risk for further injury by continuing to pitch with a torn ACL is the meniscus since the ACL prevents rotational movement. As your knee becomes more lax the femur can rotate on top of the tibia and cause shearing forces on the meniscus leading to further meniscal damage. The classic test to check for an ACL tear is a Lachman's test which is done by bending the knee and fixating the foot then seeing if the tibia will sublux forward from the femur. Yet often this is difficult to assess due to either swelling or patient guarding by contracting the hamstring which can prevent the tibia from moving forward, so there are reasons why a definite diagnosis could not be made immediately.