SuperSport recruited the Zambian forward from Power Dynamos, handing the 19-year-old a three-year deal following a successful trial in Tshwane.
Lungu, who scored 16 goals last season, made his Absa Premiership debut as a 75th minute substitute in Matsatsantsa's 1-0 win over AmaZulu at Lucas Moripe Stadium on Wednesday.
"He is a young star, [turning] 20 years old. He was playing for Power Dynamos in Zambia," Tembo says of Lungu.
"We are trying to bring in youngsters with potential.
"He is actually a striker but we are trying to convert him to a winger.
"You gonna see a lot from him. He is a quality player. He is typical goal scorer, he scores goals.
"I am telling you, you are gonna see a lot from him. He is a quality player."
United have also added Phumlani Ntshangase, Bongani Khumalo, James Keene, Washington Arubi and Luke Fleurs to their roster for 2018/19.
Tembo's charges next face Orlando Pirates at Orlando Stadium Stadium in the MTN8 quarter-finals on Saturday (20h30).