The 30-year-old is one of the unsung heroes of a Northampton side aiming to become the first team to go through an entire Heineken Cup campaign unbeaten.
And while tournament favourites Leinster can boast a dazzling array of Irish talent in their matchday 23, head coach Joe Schmidt is fully aware of the damage this particular Dubliner could have on his side's title aspirations.
"James doesn't seem to put a foot wrong," said Schmidt.
"He's a big strong lad who gives them a lot in midfield. Trust me, I've had a fair look at him this week.
"Specifically there is good footwork in his late changes of direction and he carries at a good height. He also showed pace to score that try in the pool stages at Edinburgh.
"He is a big man and when you pack that kind of punch and can get your shoulder in defensively, it makes a good player."
Such praise seems more than justified when you consider what an integral part Downey has played in Northampton's rise from the depths of demotion from English rugby's top flight to the pinnacle of the European game in just four seasons.
The 30-year-old has played in all eight Heineken Cup matches this term, as well as featuring in all seven when Saints were knocked out at the quarter-final stage last season.
Downey played an equally vital role when Saints marched all the way to Amlin Challenge Cup glory in 2009 and his overall appearance stats are just as impressive. Since arriving at Franklin's Gardens in the summer of 2007, Downey has made 119 appearances for the Club. With that figure set to rise to 120 this Saturday, his average will be a substantial 30 games a season.
And while Northampton's climb back towards the top of the continental tree has been startling in its speed, Downey's own rise has been just as remarkable.
Prior to joining a Saints side preparing for life in England's second tier, Downey was out of a job. Released by Calvisano after two seasons in Italy, Downey had previously been deemed surplus to requirements at both Leinster and Connacht and had also spent time with Munster as he failed to make a name for himself in his native land.
"Sometimes I have to pinch myself," added Downey.
"Especially when I went to Italy, I certainly would not have thought I would be in this position now.
"I was released and didn't have a job for a month, I was thinking about giving up on it. I have come full circle, from being unemployed to being in a Heineken Cup final.
"Things happen for a reason. Dave Quinlan got injured here and a door opened for me, and it has worked out for both of us. Hopefully I can repay Northampton tomorrow."
Few at Franklin's Gardens would argue that Downey hasn't paid them back already but to deny all of Dublin European glory would certainly make him even him even more of a fans' favourite in his adopted home.
To do so, however, Downey and midfield partner Jon Clarke face the toughest of tasks: shutting down one of the game's greatest centre combinations.
Brian O'Driscoll, who was today passed fit to start for Leinster in Cardiff, and Ireland team-mate Gordon D'Arcy have starred for club and country for over a decade. And while the whole world knows of their extensive credentials, Downey is more aware than most having trained and played with the pair early on in his career.
"There is no doubting Brian's ability, he is simply outstanding. His quality was ridiculous in training.
"He has superb skills, superb hands, he is the modern centre and has been the best centre in the world for the last few years.
"His partnership with Gordon is the best centre partnership in Europe and I look forward to going up against it.
"Gordon and Brian are the benchmark for centres at Six Nations level and Heineken Cup level."