Pjaca came off the bench to break his duck for Juve and put them ahead in the 72nd minute against 10-man Porto - who had Alex Telles sent off for two yellow cards in quick succession in the 27th minute - by lashing into the bottom-left corner.
His fellow substitute Dani Alves made it 2-0 two minutes later, giving Juve a commanding advantage going into the return leg in Turin on March 14.
Speaking about Pjaca, Allegri told Mediaset Premium: "I already saw from Saturday that he was changing attitude and he understood that some things are not enough when playing in Italian football.
"He is changing his attitude and already has the technique, pace and talent. He is young and we need patience with young players.
"The team did well in terms of character and we were only under pressure for five minutes, then controlled the game both with 11 against 11 and 11 against 10.
"The lads were patient, tried to widen the opposition and we brought home an important victory. It could've been 3-0, but this is fine."
Pjaca is hoping for more time on the pitch following the goal, but is willing to wait for his opportunity.
"I am very happy for the goal, but above all for the victory, as it is very important for us," Pjaca said.
"In the second leg we need to be very careful, because in football you can never tell what will happen.
"Of course I want more playing time, but I continue working the way I have been. The coach makes his choices and I am waiting for my moment."