"Chelsea have made a lot of signings but for me, he's the best," declared Robbie. "He's a class act and it's a great move for him."
Unfortunately, Robbie is unlikely to come up against the winger at Stamford Bridge on Saturday as he recovers from an ankle injury.
Nevertheless, the pair have come a long way since first breaking into the Irish ranks.
Robbie, of course, had big-money moves to Inter Milan and Leeds before settling at the Lane - all before the age of 22.
Duff, meanwhile, cost Chelsea a staggering £16m from Blackburn Rovers and remains the biggest signing of Roman Abramovic's £100-plus summer.
With that friendship, it's no surprise to find out that Robbie was one of the people that Duff telephoned for advice before joining the Stamford Bridge set.
"Me and Damien got into the Irish team at the same time, I was 17, he was 18," said Robbie of 'Duffer'. "We always room together.
"He's a good friend and and great lad and I speak to him as often as I can.
"He's moved to Chelsea and he asked me at the time what London was like and all that.
Ask Robbie what exactly Duff brings to the party and he'll point out that it's more of a case of what he doesn't bring.
"He's got everything. He scores goals - I know he always talks about scoring more, but he scores a few goals, pace is scary, off the mark I can't think of anyone as quick with the ball.
"He can beat players at will and is a great crosser of the ball. Put it this way, Chelsea have got a hell of a signing.
"I know it was hard for him to leave Blackburn because he was there as a young lad and I know how much he loves that club but how can you turn an opportunity like that down?
"I've already heard that the Chelsea fans love him, he's a crowd-pleaser, bundles of skill, a great talent.
"He's a great lad and a world-class player."
Robbie Keane doesn't hesitate as he lavishes praise on his great friend and Irish colleague Damien Duff.