The results of a recent study by Grant in the journal of Arthroscopy showed that "Outcomes were better if the ACL was reconstructed and reconstruction was delayed to allow a return of knee range of motion. In many cases, this delay may allow the MCL to heal. MCL repair or reconstruction may be required if valgus instability remains after an appropriate rehabilitation period." It concludes that "ACL reconstruction should be performed in a subacute time frame once full motion has returned. Valgus instability should be assessed at that time and MCL repair or reconstruction performed in those patients with persistent valgus instability."
Partial ACL injuries pose a problem of their own. Is the knee stable enough to return to high level sports without ACL reconstruction? Patients that are able to return sports without surgery are considered "copers." In 2011 Tjoumakaris published an article in the American Journal of Orthopedics, "Partial Tears of the Anterior Cruciate Ligament: Diagnosis and Treatment." The article states that the "Natural history studies following patients with these injuries have demonstrated that fewer than 50% of patients return to their preinjury activity level. Several studies have also documented that progression to complete rupture is a common outcome for patients who want to return to an active lifestyle."
Although the research isn't conclusive, the evidence supports subacute ACL reconstruction with concurrent MCL repair/reconstruction if valgus laxity persists. In the case of partial ACL injuries, reconstruction may be the best option due to their high likelihood of progression to complete rupture and the poor rate of return to pre-injury levels of sports participation.