Leeds turned to Gnonto in the dying hours of transfer deadline day in the summer, having been snubbed by their priority targets Cody Gakpo and Bamba Dieng.
The Whites initially wanted to place Gnonto with the Under-21s, but he has forced his way into the first team mix and was again in good form against Aston Villa on Friday night.
Gnonto provided the assist for Patrick Bamford to score for Leeds in their 2-1 loss and McManus was impressed with what he saw.
He does not feel the Italy international will be at Elland Road for long, but does think Leeds will make a big fee when he is sold.
McManus wrote on Twitter: “Gnonto wow what a talent for Leeds. So impressed watching him.
“Wont be there long if continues to put in performances like tonight.
“Can’t believe he is only 19.
“Leeds are going to make a fortune.”
Gnonto spent time in the youth ranks at Italian giants Inter at the start of his career and then headed to Switzerland to make his senior football breakthrough at FC Zurich.
