Saints have slapped a £100m price-tag on the teenage winger as England's biggest clubs circle ahead of the summer market.
"He's the most fearless player I've ever worked with," Asghar tells BBC Sport.
"He's totally unique to anything I've seen before. He's got raw physicality, pace and power, is an unbelievable ball-carrier and is always impacting games.
"In a generation of football where it's very much about limited touches and moving the ball quickly Tyler can move it as quickly by carrying the ball rather than passing it and that's what makes him such a special talent.
"Every game (with the Under-21s) he would always have four or five moments that would almost take your breath away.
"He is really sure of his ability. He wouldn't be fazed by the opponent or situation, he played with fearlessness."
Asghar also said: "Your job as a coach is to try and influence his behaviour and that's what the talented ones will make you do.
"When you spoke to him it was clear what he wants. It could be quite blunt in terms of the training content he likes, how he wants to play the game. That's why you can see him handle the transition to the big stage really well.
"He knows what he wants, he knows what he can bring and some things that can be maybe portrayed as aloof.
"The guys who have worked with him definitely know that's not the case. He's a good boy, he wants to listen, he'll take things in."