The Portuguese wore that famous jersey throughout his six seasons as a Red between 2003 and 2009, but it was not available when his transfer back to the club was confirmed last month.
Cavani wore the no.7 shirt throughout his successful debut season at United and the incredibly popular striker began this campaign with it too, during his substitute appearance at Wolves.
However, after Daniel James was sold to Leeds United on transfer deadline day, the no.21 shirt was suddenly available and El Matador graciously took it on so that Ronaldo could therefore wear seven.