Not all patients with an ACL tear injury are candidates for a surgery for an ACL tear injury. This depends on a number of factors which include the grade of the tear, the age of the patient, the level of activity prior to the injury, and the patient's will to change their lifestyle to heal ACL tear injury and reduce activity levels.
A professional athlete with a complete ACL tear who wants to go back to sports again cannot expect a positive outcome from conservative treatment. Conservative treatment can help a lot of patients, however. The patient can use rehab exercises to heal a torn ACL without surgery. A patient with a partial ACL tear injury who is ready for lifestyle changes and is not involved in demanding activities is a great candidate for conservative treatment.  Even if the conservative treatment fails, the patient can get surgery later. 
A patient with an ACL tear injury will be unable to fully bear weight shortly after the injury, due to pain. Also, swelling of the knee joint is very common and can limit the motions in the knee. Although a brace is now a common practice after ACL reconstruction, it can be used in the case of conservative treatment .
In some cases, a functional brace is used for two to three months to allow a torn ACL to heal. This works well, but only in case of partial ACL tears in low athletic-demand patients.  The brace is used to reduce the pain shortly after the injury, to increase the stability of the knee, reduce the possibility of injuring an already partially torn ACL, and to provide the patient a chance to fully heal.
Once the pain and swelling subside, the patient is ready to continue with rehab exercises to heal a torn ACL without surgery. It is important not to delay rehabilitation for too long because it can have a negative effect on the final results. 
The main role of rehabilitation exercises is to increase the strength of the muscles surrounding the knee so they can keep the knee joint stable. That increased stability will prevent excess movements of the knee and the site of the ACL injury.
It is important to increase the strength of the anterior and the posterior group of muscles (hamstrings and quadriceps) so that they stabilize the joint.
Quadriceps Strengthening Exercises After An ACL Injury
The quadriceps is a muscle very susceptible to inactivity and it will start to atrophy and lose strength shortly after immobilization. Most patients will use a brace right after the injury to prevent pain. But that brace will immobilize the knee, and the quadriceps will start to deteriorate. So, the patient should start with quadriceps strengthening exercises right after the injury, even while they are still in a brace.
- Lay on your back with your leg extended in the brace and contract the quadriceps. Hold it contracted for five seconds and then release it. Take a two-second break and then repeat the action. You can repeat this 15-20 times.
When you feel more secure and don’t have that much pain you can add another quadriceps strengthening exercise to your daily routine.
- Lay on your back with your leg extended. Pull the foot towards you so you can feel the tension in the lower leg, contract the quadriceps and elevate the extended leg. Hold the leg elevated for five seconds and then slowly bring it back to the bed, relax the quadriceps and the foot. Take a five-second break and repeat this action again nine more times.
These exercises will help your quadriceps not to lose strength while immobilized. Stronger quadriceps mean increased stability of the knee joint.
Hamstring Strengthening Exercises
Hamstrings are very important for knee stability. Even though hamstrings are not that susceptible to immobilization, it is important to keep them strong. For some of the exercises you will need help, but there are some which can be done with help of an ankle weight or resistance band.
- Lay down prone in bed with your legs extended. Use an ankle weight or resistance band while trying to flex the knee. You will feel tightness in your hamstrings. Repeat this exercise 10 to 15 times and then take a break. It is easier if you have someone to help you with your exercises. Your assistant can help you and keep you motivated.
- Lay prone on the bed with extended legs. Now flex your leg in the knee up to 30 degrees and ask your assistant to place their hands on your heel and provide resistance while you are trying to flex your leg even more. You will feel the contraction of your hamstrings and then hold that position for five seconds. After that relax, and bring your leg to the starting position. Repeat this process ten times.
In conclusion, there are a number of rehab exercises to heal a torn ACL without surgery. If you suffered an ACL tear injury, consult your physician and discuss whether you are a candidate for a conservative treatment.