Nyoni made the switch from Leicester official towards the end of August, with the Reds having tracked the teenager for a number of months.
Him and fellow new arrival Amara Nallo have immediately slotted in the under-18s setup, making their competitive debuts in a 6-1 loss to Crystal Palace last weekend.
The international break brought the opportunity for a friendly at the AXA Training Centre, as Liverpool helped Southend in their preparations for the new campaign.
It resulted in a 10-0 victory for the young Reds, with Nyoni scoring a hat-trick and three other player netting braces.
Ranel Young, back from injury, was one of those to strike twice, with Kareem Ahmed and Fola Onanuga also doing so while Wellity Lucky scored once.
Though it came against lower-level opposition in the youngsters from Southend, it will be a major boost to the U18s ahead of their trip to Blackburn on September 16.
That will particularly be the case for Nyoni, who manager Marc Bridge-Wilkinson believes will “take a lot” from his successful afternoon.
“It was a really pleasing performance and we played really well,” he told the club’s official website.
“We scored some excellent goals and it was nice to see the work we’ve been doing on the training pitch really pay off today.
“Trey got his first goals in a red shirt, he will take a lot from that, and it was great to have Ranel back.
“Fair play to Southend United as well. They never gave up even with the high scoreline, they kept working hard and we thank them for coming to Kirkby.”
