The former Barcelona striker told Marca: "Cristiano already knew the club but that was at a different stage and now he has adapted in an impressive manner.
"From the start, he has scored goals as he always does and had no problems adapting."
Messi also weighed in on United's recent dip in form, with a 4-1 defeat to Watford last time out leaving them seventh in the Premier League table and ultimately costing Ole Gunnar Solskjaer his job as manager.
The PSG forward is confident that the Red Devils' fortunes will improve as the season progresses given the individual quality in their ranks, as he added: "United is a very strong team with great players.
"The Premier League is a very difficult competition and an equal one, and things can turn around many times.
"After December, a lot changes and anything can happen."
The Argentine's best years with the Blaugrana came when Ronaldo was on Real Madrid's books as the pair competed for Golden Boot and Ballon d'Or awards season in, season out.
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Asked if he misses pitting his wits against the Portuguese on the domestic stage, Messi replied: "Time has passed since we competed in the same league. We not only competed on an individual level but also on a team level for the same aims.
"It was a very beautiful stage for us and for people because they enjoyed it very much. It's a beautiful memory that will remain in football history."