After leaving Toon in the summer, Benitez told The Athletic that the club did not want to invest in the academy.
A graduate himself, Matty Longstaff scored the winning goal on debut against Manchester United on Sunday.
In a post on his personal blog, Benitez wrote: "I was really pleased to see Matty Longstaff scoring the winning goal for Newcastle , he was great when he was training with us and it was a good decision from Steve Bruce to make him play.
"I was also happy to see a few Geordies playing together at the same time; Carroll, Sean and Matty. Surely it shouldn't be difficult to see the benefits of investing in the Academy?
"£4m every year for 10 years means £40m (one player); one 'Longstaff' every year for ten years seems good business to me."