Sorry to say that even if you had the pup for 5 days an ACL Tear is purely environmental - eg { your fault }
I personally would never get ACL surgery on a pup that age , you need to take him/her to a physiotherapist / acupuncturist for advice.
Surgery will ruin the growth plates.
It's going to be a lot of work for you to fix this successfully however I have done it myself so it's def possible with dedication.
Good luck , but don't blame the breeder.
Also buying crossbreds are buyer beware..