Ronaldo and Messi are regarded as two of the greatest footballers of all-time following their exploits for club and country.
Portugal and Juventus superstar Ronaldo – a five-time Ballon d'Or winner with 32 major trophies – matched Ali Daei's international men's record for most goals (109) following his two-goal haul in the 2-2 draw with France at Euro 2020.
Record six-time Ballon d'Or winner Messi has won a club-record 34 trophies at Barcelona, while he equalled Javier Mascherano for the most Argentina appearances in their 1-0 Copa America victory over Paraguay.
As Austria prepare for Saturday's Euro 2020 last-16 showdown against Italy, Arnautovic hailed Ronaldo and Messi.
"When you mention [Cristiano Ronaldo's] name, we aren't talking about a human being," Arnautovic said. "That’s it. What am I supposed to say about it?
"There are two players who are not from this world. They are from far away and just came here for a while to succeed in every league and take all the trophies with them. And then they are leaving again.
"Those are Ronaldo and [Lionel] Messi."
This will be Austria's first ever appearance in the knockout stages of the European Championship, and their first in the knockout stage of any major tournament since the 1954 World Cup, when they reached the semi-final.
Austria have lost four of their six games in London in all competitions (W1 D1), with this their first visit to the English capital since a 7-0 friendly defeat in September 1973 (against England).
The nation's only previous game in London which was not against England came in the 1948 Olympic games, a 3-0 loss to Sweden.
David Alaba created nine chances in the group stages, at least four more than any other Austria player. Four of these came in Austria's 1-0 win against Ukraine last time out.