That's a nonsense theory from that commentator, whoever he is.
First of all, if it was true then why did Conte play Del Piero yesterday? What if Del Piero won us the match? Then again Conte would have had to play him, wouldn't he?
Conte would kill to win the scudetto and if he thinks that Del Piero is the answer to winning the scudetto, then he'll play him non stop.
Then Conte would say:"I told you he'll help us, I know how and when to use him" and yada yada yada.
But if Ale starts, shines and we lead 2-0\3-0 in HT, people will say:"The fucking fuck...where was this guy before?", and then Conte will have to play him much more.
For conclusion- if you give him 10 mintues and he scores, you'll have to give him 10 mintues again. If he starts and shines, you must start him again. That's the difference. You can make people forget quickly about 10 mintues and one goal, but not about an impact a player had on the team for a whole match.