That's not true - every tear is different. A doctor friend of mine tore her ACL while skiing on her first run of the day up at Jay Peak, and kept going. She knew she'd done something, but it didn't bother her at all, and when she finally had it looked at she said "oh that makes sense" and only did rehab. No surgery or anything.
Another friend just did his two weeks ago playing rec soccer. Another instance of just landing wrong and feeling a slight tweak, and then some discomfort. After two weeks he finally went to the doctor and got an MRI, and now he's deciding whether or not he wants surgery.
Not all bodies are the same, and not all tears are the same. Roberto Baggio came back from an ACL in 4 months; other guys have taken 1.5 years.