Ronaldo praised Cavani for the “incredible gesture” of handing over the No 7 shirt as the Portuguese forward returned early from the international break.
Ronaldo, 36, rejoined United from Juventus this summer, returning to the club 12 years after leaving for Real Madrid.
Ronaldo wore the No 7 during his first spell at United, during which he made 292 appearances and scored 118 goals, helping the club win nine trophies including three Premier League titles and the 2008 Champions League.
However, with the season already under way, Cavani retained the No 7 jersey from last season and wore it in the 1-0 win over Wolves on Sunday.
“I wasn’t sure if it would be possible to have the number seven shirt again, so I would like to say a huge thank you to Edi for this incredible gesture,” Ronaldo said on United’s Twitter account.