Messi and Ronaldo have won six and five Ballons d'Or respectively and may face each other next Wednesday when Juve host Barcelona in the Champions League, though the Portuguese must first be cleared for action after being diagnosed with coronavirus.
The pair have previously met in five Champions League encounters, with Messi 3-0 up on Ronaldo in terms of goals scored.
Asked to outline the differences between the two players ahead of that high-profile match in Turin, Pjanic said: "They are both incredible, in the truest sense of the word.
"Only in a few years, once they've retired, will we realize the level they reached in their football and the type of unbelievable, unreachable talents they are."
Ronaldo has scored and assisted a combined 69 goals since joining Juventus in July 2018.
"He is an absolute champion. An incredible worker who takes care of every detail," Pjanic told Tuttosport. "He is obsessed with goals, statistics, victories - a true winner.
"He is someone who has won wherever he has gone, a leader, but also a person who places himself nicely within the group."
Pjanic spent two seasons playing in the same side as Ronaldo prior to joining Barcelona, where he is now linking up with Messi.
"Lionel is awesome. It is difficult to describe - what you see in the game makes him an alien himself," the former Roma player said.
"But when you see him up close, in training, you realize that he is always like this - it is his natural condition.
"He amazes you with his technical gestures - it is not something normal. Every time he has the ball something happens."
Should Ronaldo be deemed fit, next week's game will be the first time he and Messi have faced off in the Champions League since a semi-final tie between Madrid and Barca in May 2011.