Ronaldo re-joined United, after spending six years at Old Trafford from 2003 to 2009, late in the transfer window from Juventus and made his return from the starting XI.
The Portuguese opened the scoring with a close-range finish after a rebound on half-time, before restoring United’s lead in the 62nd minute after Javi Manquillo’s equalizer.
Ronaldo’s display drew praise for United boss Ole Gunnar Solskjaer, who labelled him “ruthless”.
“Cristiano senses the big moments and is ruthless. He is clinical,” Solskjaer said.
“He has lifted the team and everyone around us. It’s just perfect for him, you see what he’s about, sensing the moment and smelling the danger but he’s also evolved into a proper team player.
“He has developed into a fantastic goalscorer. He played a very good game, dropped in and linked the play well.”
Elsewhere on Saturday, champions Manchester City got past Leicester City 1-0 thanks to Bernardo Silva’s 62nd-minute goal, making it three wins on the bounce.
Arsenal broke their duck with a 1-0 home win over Norwich City with Pierre-Emerick Aubameyang netting the winner.
Tottenham’s perfect record was ended at the hands of Crystal Palace who triumphed 3-0, scoring three goals in the final 20 minutes.
Chelsea cruised past Aston Villa 3-0 with Romelu Lukaku scoring a double, while Michail Antonio was sent off as West Ham drew 0-0 at Southampton.
There were defeats for the newly promoted sides as Wolverhampton won 2-0 at Watford and Brentford lost 1-0 at home to Brighton.