Barcelona superstar Messi and Ronaldo have won five Ballon d'Or awards apiece, with ongoing debate as to who is the greatest player of all time.
Asked about Messi at the Laureus Awards in Monaco, Capello – who won two La Liga titles in two spells with Madrid – told reporters: "Leo Messi is not a footballer, he is a genius and the only genius in world football.
"He has not won much silverware with Argentina but that is because he cannot do it all alone."
Capello did laud Ronaldo following his move to Juventus, where the former Madrid star has flourished this season.
After joining in a €112million deal ahead of the 2018-19 campaign, Ronaldo has scored a Serie A-high 19 goals and 22 in all competitions for the Italian champions.
"Cristiano has woken up a lot of Juventus players," ex-Juve player and coach Capello said. "Bringing him to the club has been a wake-up call for many players and he has made them improve.
"He is very important for Juventus because having someone as good as him at training every day and someone who takes care of himself in the way Ronaldo does, is hugely significant for a team that was coasting.
"It is obvious that he is desperate to win the Champions League with Juventus."
Capello retired from coaching in 2018 after departing Chinese side Jiangsu Suning.
Messi and Barcelona are set to take on Lyon on Tuesday in the round of 16 of the Champions League.
Barca currently sit atop La Liga, with Messi's 22 goals in 22 games, with a seven-point lead on Atletico Madrid and a nine-point advantage over Ronaldo's former club Real Madrid.
Ronaldo meanwhile is scheduled to return to his former city on Wednesday for a clash with Atletico as Juventus continues their Champions League push.
Juve, meanwhile, maintain a 13-point lead over Napoli in Serie A with Ronaldo leading the league with 19 goals in 24 league games.