Joseph O'Brien believes it could be a "big ask" for Investec Derby favourite Dawn Approach to see out the mile-and-a-half trip at Epsom on June 1.
O'Brien, who won the Derby last year on Camelot and is set to ride Battle Of Marengo from his father Aidan's squad of possibles, spoke of his admiration for Jim Bolger's 2000 Guineas hero but questioned whether the Derby distance would suit.
Speaking on Channel 4 Racing, O'Brien said: "You can't fault him, he's tough and he tries. It's a big unknown stepping up to a mile and a half, he won over five last year and he won a Coventry.
"To get a mile and a half this year is a big ask."
The Ballydoyle stable has dominated the trials for the Epsom Classic, with the one blip coming in the Dante Stakes at York.
Battle Of Marengo won the Derrinstown Stud Derby Trial at Leopardstown, and while his manner of victory did not impress everyone, the Galileo colt beat a previous Group One winner in the Bolger-trained Loch Garman.
O'Brien said: "We're very lucky with some nice colts, nice three-year-olds and a couple of them won nicely, albeit in below-average trials you could say. Battle Of Marengo is a tough horse, he keeps winning and gets the job done. He might not be very stylish but he's tough and he stays.
"Nevis won well in Lingfield and Ruler Of The World and Magician in Chester, so we have a couple of nice horses to go to war with."
Asked what horse he is likely to ride, O'Brien said: "Strictly on form you can't get away from Battle Of Marengo. He beat a Group One winner in his trial and beat him nicely by a couple of lengths without having a hard race.
"In all the other trials I think you would go a long way to find a Group One winner other than the couple of winners of them. He's tough, he's genuine and he's had plenty of racing and at the moment he looks one of the principals."