The striker completed a season-long loan move to Old Trafford on Monday and the former club favourite is confident his arrival will provide a boost to the misfiring team
The Colombian completed his season-long switch from Monaco on transfer deadline day after Louis van Gaal beat off interest from Manchester City and Arsenal.
United will pay a €12.5 million loan fee and a significant portion of his €380,000-a-week wages but Ronaldo feels Falcao - who was also linked with a move to Real Madrid - is a coup for his former club.
"He is a very good player, I think Manchester United have done well to sign him," Ronaldo told reporters. "We are talking about a top player here and a fantastic signing."
Falcao became United's sixth major summer signing after Luke Shaw, Ander Herrera, Marcos Rojo, Angel Di Maria and Daley Blind, but they are yet to record a competitive win under new boss Van Gaal.
A home loss to Swansea on the opening day of the Premier League season was followed by draws with Sunderland and Burnley and a 4-0 Capital One Cup humiliation at the hands of lower-league MK Dons.
And Ronaldo has backed Falcao is improve United's fortunes, with his arrival proving a boost for his new team-mates.
He continued: "Manchester United have not had a good start with only two points, so the squad needed someone of Falcao's quality."