Lazio will reportedly cover more than half of Pereira's wages for the season.
The 24-year-old fell out of favor with United manager Ole Gunnar Solskjaer and started just four times in the Premier League in 2020. Pereira signed a long-term contract with the Red Devils last summer but ultimately fell to the wayside when Portuguese international Bruno Fernandes arrived in January from Sporting Lisbon.
Pereira was previously linked with moves to Benfica and Valencia, where he spent the 2017-18 season on loan.
Pereira, who's capable of playing centrally and out wide, will offer Lazio boss Simone Inzaghi depth in attack.