Honestly, if you are worried about this. Then you aren't really enjoying the manga all that much. However, I personally don't think Sasuke obtain the Perfected Sussano'o by using someone else's power. Sasuke have shown to have the best, natural skill with the Sharingan. Mastering the normal 3-tomoe Sharingan at a young age, gaining and fully control the MS before becoming an adult and obtaining and mastering ( more or less ) the EMS only a few short months/weeks later. This is the quickest mastery of the Sharingan in the Uchiha history. ( Faster than Madara, however Madara might have more knowledge than Sasuke ) So I personally think that Sasuke knew he was already able to use the Perfected Sussano'o prior to entering the battlefield.