The ACL is a stabilizer of the knee. It is quite important for the bio mechanics of the knee. Knee is not a simple hinge, but a complex bicondylar joint. Many studies have shown that ACL injury if left untreated may lead to early arthritis and injury to the menisci. But in simpler terms, if the knee buckling and feeling of give way is severe, better to reconstruct. Stem cell therapy helps in regeneration of tissue, but it's efficacy if still not fully documented. But it will have no immediate stabilising effect. I don't know any studies which have shown ACL regeneration with stem cell therapy.