People hear "getting an MRI on his knee" and immediately jump to "getting an MRI, possible torn ACL". The earliest tweets said it was just to confirm that all he did was hyperextend his knee. That seems to be the case.
However, you can walk pretty much completely fine with a torn ACL. Though, I doubt after a visit to the doctor's office they'd let you just walk around until surgery.
My cousin tore his ACL and he actually continued to play like pick up games and his leg would just give out so he went to the doctor and indeed he tore his ACL.