There are several tests physicians use to check the integrity or strength of the repaired ligament. One of these is the pivot-shift test, in which the doctor applies pressure to the lower leg just below the knee. A "tight" joint will not shift or move with this test.
Your sense that the leg is not steady or stable enough to hold you may be caused by muscle weakness. When the replacement tissue for the torn ligament is taken from below the kneecap, the quadriceps muscle on the front of the thigh may lose strength. You may need some additional strengthening exercises. Contact your physical therapist for a follow-up evaluation.