It is a measure of Kerr's work - and Duff's enormous contribution - that Fifa adjudicates that the Irish sit just three places below their opponents in those rankings. Duff will face two of his Chelsea team-mates - Tiago and Paulo Ferreira - and Kerr was quick to praise the progress Duff has made at Stamford Bridge.
"It was a huge challenge for him at Chelsea," Kerr said. "I remember when he went there he felt that it might be a little hard for him. But he has risen to that challenge and shown he has the mental strength and physical ability to survive as well as the technical ability."
His value to Chelsea, especially with Arjen Robben injured, has now rocketed beyond the pounds 17m they paid for him. When that deal was done, Blackburn Rovers privately warned that Chelsea would not get much more than 30 games a season out of Duff, such was his propensity to injury. The 25-year-old Irishman is proving that wrong, although his club manager Jose Mourinho will hope that he does not play the 90 minutes tonight.
Duff will want to. It is his way. "He's a top player," said Kerr, who first plucked the Dubliner as a 17-year-old to play in the Under-20 World Cup finals in Malaysia. His contribution since then has been immense. Kerr said: "His performances have been fantastic. He is prepared to do whatever is required for the team. He is a special talent and he looks like he is right on top of his game."
One frustration has been his lack of goals - just six - but the triumvirate that is emerging with Duff, Robbie Keane and Andy Reid is, Kerr declared, "some of the most exciting stuff we have seen for a long time". All three, he said, are "street-type" players - shrewd and tricky on the ball.
The Irish go into this match without Roy Keane and Stephen Carr but are protecting a record of just two defeats in 24 fixtures under Kerr. Portugal coach Luiz Felipe Scolari was asked for a prediction. "A draw," he said. He's likely to be right.
REPUBLIC OF IRELAND (probable 4-4-2): Given (Newcastle United); Finnan (Liverpool), Cunningham (capt, Birmingham City), O'Brien (Newcastle), O'Shea (Manchester United); Duff (Chelsea), Holland (Charlton Athletic), Kilbane (Everton), Reid (Tottenham); Morrison (Birmingham), Keane (Tottenham).
PORTUGAL (4-2-1-2-1): Ricardo (Sporting Lisbon); Ferreira (Chelsea), Andrade (Deportivo La Coruna), Caneira (Valencia), Rogerio Matias (Vitoria); Petit (Benfica), Tiago (Chelsea); Deco (Barcelona); Ronaldo (Manchester United), Simao (Benfica); Pauleta (Paris St-Germain).
Referee: M Messias (England)