There are already calls for the attacker to be promoted to the full Italy squad.
"My primary objective with the Under-21 side is to ensure that as many young players as possible go on to Cesare Prandelli's full national side," Mangia told Radio CRC on Monday.
"The Coach will make his decisions, but there is a total understanding between us.
"However, Insigne is a boy who needs to grow and we shouldn't put too much pressure on him right now.
"Lorenzo is a splendid youngster and he's trying to showcase everything he can do to achieve something important.
"He's growing very quickly, but he needs time and patience."