Barnes says the skills of Lyon's Juninho and former Real Betis star Marcos Assuncao mean the United player still has some catching up to do.
He said: "Ronaldo is a great free-kick taker but for me he relies completely on power and he may feel he should start to vary what he does.
"If he wants to add to his armoury, he should watch the way Juninho, in my view the best at this art in the world, applies sophistication as opposed to power to bend it like crazy even from 30 yards. Assuncao is almost as good - I once saw him score a hat-trick of free-kicks for Real Betis - and he did not employ power.
"Skill makes the difference and Ronaldo has so much of it - but right now he is still behind Juninho and Assuncao."