The main differences between an ACL and an MCL tear are a significant amount of swelling often occurs when you injured the ACL as well as the amount of instability when walking after the ACL injury. Generally speaking, the MCL has a much higher healing potential than the ACL. Torn ACL patients are more likely to rely on crutches or knee brace for walking after the injury. ACL and MCL tears can also be combined with multi-ligament injuries of the knee. A grade 3 MCL tear can be generally occurred along with a torn ACL.
Technically speaking, you can walk without the ACL if you are only participating in straight line activities such as walking or cycling. However, if your ACL is completely torn, continue to walk may damage other parts of the knee and affect knee function for a lifetime which can lead to osteoarthritis if not treated correctly.