Speaking to Sporza, Sagan said that Boonen was one of the favourites to win Paris-Roubaix, and that it would be great if the Belgian could end his career on a high in his final race.
"For sure Tom Boonen is a favourite to win Paris-Roubaix," Sagan said at Tirreno-Adriatico where he has won two stages.
"I will be very happy if he could win Paris-Roubaix again. He's a big name, a big rider, and it would be a great win for him."
Four-time Paris-Roubaix winner Boonen plans to retire after this year's edition of the race, drawing a close to a 16-year professional career that has also included six stage wins and a green jersey at the Tour de France, three Tour of Flanders victories, and a World Championship win in 2005.
One race that neither Boonen nor Sagan have won is Milan-San Remo, which Sagan describes as a "very strange race".
Both riders will take to the start line in Milan on Saturday, with Sagan being the sole leader of his Bora-Hansgrohe team, while Tom Boonen could find some internal rivalry with fellow Quick-Step Floors rider Fernando Gaviria, who out-sprinted Sagan in the penultimate stage of Tirreno-Adriatico on Monday.
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