Nallo, 19, will spend the remainder of the season with the four-time defending Finnish champions, whose regular campaign concludes in November before potential championship play-off rounds should HJK look to retain their title.
The London-born centre-back joined Liverpool's academy from West Ham in autumn 2023 and has quickly emerged as one of the club's most promising young defenders, captaining both the Under-21s and the side's UEFA Youth League campaign.
He has made three senior appearances for Liverpool, including 75 minutes in a 6-0 Champions League win over Qarabag last season, and was named among the substitutes for a Premier League fixture against Newcastle in January, while also representing England at youth level.
"The intensity in training was really high. It's great to play with quality players. The standard is high, I really like that," Nallo said of his first sessions with his new club.
"HJK is the biggest club in Finland, so of course I'm excited. I've come here to win, I want to win as many games as possible and play as much as I can.
"Hopefully we can play for the championship and win the cup."
HJK sporting director Petri Vuorinen said the club had moved quickly to add defensive depth following the departure of Brooklyn to Derry City.
"With Brooklyn's move to Derry City, it was important for us to quickly add width to the centre-back department, and things have progressed well with Amara, on schedule and according to plan. Amara brings us a good option in defence for the rest of the season. He is a promising young centre-back who brings strength and duelling power to the important matches this autumn," Vuorinen said.
Elsewhere, Liverpool remain among the clubs monitoring Pietuszewski, the 18-year-old Poland international who only joined Porto from Jagiellonia Bialystok in January for a reported £8.5m.
The winger has scored three goals and registered four assists in 16 league appearances for the newly crowned Portuguese champions, whose hierarchy are keen to increase his £51m release clause and tie him to a contract extension amid growing interest from across Europe.
It remains highly unlikely Liverpool would consider meeting that release clause figure, though a compromise fee could yet be found, with the club's more immediate priority being to replace Mohamed Salah following his departure this summer.