The Premier League's all-time top scorer Alan Shearer was stunned by Ronaldo's performance in United's 3-2 win, Reuters reported.
"You just have to sit there and say 'wow' and applaud the guy," Shearer said on Amazon Prime Video.
"It's very difficult to get to the top but it is staying there too. You have to get up in the morning and go again and the whole world is looking for you to perform every week. It's just phenomenal what he's done."
Former Arsenal forward Thierry Henry said he would need "two lives" to better the 36-year-old's latest record.
Ronaldo has scored 686 club goals for Sporting Lisbon, United, Real Madrid and Juventus, and 115 goals for Portugal.
There is some debate over his status as the most prolific scorer in men's football, with Brazil's Pele and Romario both claiming over 1,000 goals, but statisticians say the Portugal forward tops the scoring charts in official competitions.
Former England international Gary Lineker said Ronaldo's latest career milestone was "utterly bonkers".
"He is indisputably one of the greatest players in the history of the game. An absolute phenomenon," Lineker said on Twitter.
Ronaldo is United's top scorer this season with 12 goals in 16 appearances in all competitions. His brace against Arsenal took United up to seventh.