Messi and Ronaldo have been by far the two greatest players of the past 15 years, and arguably the two best of all time, having accumulated dozens of trophies between them, 11 Ballon d'Or wins, and hundreds and hundreds of goals over the years.
Over that time, the debate has raged on about who is better, and now Koeman has had had his say.
But, he reckons it is hard to choose between them.
“It’s fantastic that these two players have been so long at this top level in the world,” the Barca boss told reporters this week.
“They are the best players, in my opinion, in the last 10-15 years. Being successful all this time is incredible. It’s a great compliment to those players.
“They are different, but they have incredible stats about scoring goals, hat-tricks, about winning prizes.
“It’s not nice to say who is better because both are incredible, I admire both players because they gave us so many nice evenings with great goals, with great titles. Let’s hope we can enjoy them tomorrow night.”
As for whether Barca will have a special plan to deal with CR7, Koeman said no individual player is singled out for attention.
“For us it is not an extra motivation because of an individual player," he explained.
“We always thought of this as an important match with two big teams meeting. The two teams are going to fight for first position in the group.
“It’s always important the best players are there. I think Cristiano Ronaldo is still one of the best, he always scores and always fights.
“Because of his daily work he is still in the elite in our sport. It’s a great compliment to him. Not just any player can do that.
“We have to defend well against him and when we have the ball that danger goes away.”
Barca are currently top of their Champions League group, having already qualified for the last-16.
However, their European form is in stark contrast to that of their performances in La Liga, as at the weekend they suffered their fourth defeat of the campaign, this time against promoted side Cadiz.