The Italian coach was full of praise for the young Dutchman, but admitted his side were not at their best in Tuscany
The Dutchman hit the winner right at the death in Tuscany as he latched onto Thiago Motta's pass before slotting just inside the post from the edge of the box to seal three precious points for Inter, who have moved up to 15th place and are now four points clear of the drop zone.
Grateful, Ranieri was quick to sing the 19-year-old forward's praises after the clash.
"He's a good boy; I've been on his side since I got here," the former Chelsea and Roma boss told reporters after the game.
"He has a great future ahead of him; I'm grooming him for different roles in the team. He's a striker by nature, a natural centre-forward, but he can also play on the wing.
"It's time to weigh things up match by match. I'm asking the players to sacrifice themselves as they aren't playing for an Inter that are playing their best football; they are playing so that Inter can play their best football."
Ranieri claims Siena made things difficult for his side, but also admitted his team were not at their best.
"We came to play a team that move well and close down on spaces, while we were sluggish," he explained.
"But it wasn't easy. Anyway, I'm happy because the lads didn't give up for one minute and we claimed our three points - that's what's important."
However, the 60-year-old coach says his side's lacklustre display is understandable following their Champions League clash at Turkish side Tranzonspor on Tuesday, which ended 1-1.
"Let's not forget that we were in Turkey on Tuesday and got back at 6am on Wednesday morning. The players who were drafted in needed to get a feel for the pace of the match. This victory was down to work and sacrifice."