Rousseau played in the first two games of his true freshman year in 2018 before suffering a season-ending ankle injury and redshirted.[4] He entered his redshirt freshman year in 2019, as a backup but eventually took over as starter later in the season.[5][6] He was named ACC Defensive Rookie of the Year after recording 15.5 sacks that season, second in the nation behind only Chase Young's 16.5.[7][8] He opted out of the 2020 season due to concerns regarding the COVID-19 pandemic.[9]
Rousseau was selected by the Buffalo Bills in the first round (30th overall) in the 2021 NFL draft.[13] He signed his four-year rookie contract, worth $11.37 million, on June 4, 2021.[14]
During Week 2 of the 2021 season, Rousseau was nominated for NFL Rookie of the Week honors with five tackles, two sacks, and two tackles made behind the line of scrimmage.[15] Rousseau recorded his first career interception on October 10, 2021, in a 38-20 Week 5 Sunday night win against Patrick Mahomes and the Kansas City Chiefs, earning AFC Defensive Player of the Week.[16][17] In his rookie season, Rousseau finished with four sacks, 50 total tackles, one interception, four passes defended, and one forced fumble.[18]
In the 2022 season, he finished with eight sacks, 37 total tackles, four passes defended, and forced fumble in 13 games.[19] His best performance of the year came in week 14 against the New York Jets, in which he recorded 2 sacks, including a forced fumble, and a pass defensed, contributing to a 20–12 Buffalo win one week after the team's leading pass rusher Von Miller suffered a season-ending knee injury against the Detroit Lions on Thanksgiving.[20]
On April 29, 2024, the Bills picked up the fifth-year option on Rousseau's contract. In Week 1, he recorded six tackles, three sacks, and a forced fumble in a 34-28 win over the Cardinals, earning AFC Defensive Player of the Week.[21]