The 27-year-old netted an impressive 46 goals in 38 La Liga appearances for Real Madrid to help the side to the Spanish league title, and Gullit will be keeping a close eye on Ronaldo this summer.
"While there are many stars determined to leave a mark in Poland and Ukraine, I will be keenly following the performance of a select few in the tournament, those whose quality should determine how their respective teams perform in the competition," Gullit wrote in a column for The Times of India.
"One of them would be Cristiano Ronaldo. He has come into his own under Jose Mourinho at Real Madrid and will be keen to replicate that form in the national colours. If he does so, that will make him a very dangerous customer. He is fleet-footed and has the ability to score from any position."
The Euro 1988 winner then went on to single out compatriots Arjen Robben and Robin van Persie as potential stars at this summer's tournament.
"The others include fellow Dutchmen Van Persie and Robben. I’m sure the World Cup final loss will still be hurting them and they will be keen to set the record straight at Euro 2012. Van Persie has been a revelation for Arsenal this season and Robben also got his scoring touch back at the right time."
Netherlands will start Euro 2012 with the match against Denmark on June 9, while Portugal meet Germany in the other Group B opener.