In 2018, he played with
Toronto FC III in the Canadian third-tier
League1 Ontario, making his debut on August 19 against
Ottawa South United. In September 2021, after a successful trial, De Rosario signed a three-year contract with
Ukrainian Premier League club
Rukh Lviv, and was assigned to the
reserves. He made his professional debut for Rukh in the UPL as a starter against
Mynai the next month on October 24. In February 2022, he left the club by mutual consent, nine days before the
2022 Russian invasion of Ukraine after United States President
Joe Biden urged all American citizens to leave Ukraine. After leaving Ukraine, De Rosario joined
Canadian Premier League club
York United on trial during their pre-season in March 2022. The next month on April 6, York announced they had signed De Rosario to a one-year contract, with options for 2023 and 2024. He made his debut in York's season opener against the
HFX Wanderers on April 7. In De Rosario's next match on April 15 against
FC Edmonton, he started his first match, in which he scored his first goal. On August 2, he was named Player of the Week for the first time for Week 17. In September, he became the club's all-time leading scorer, when he scored his 12th goal. At the end of the season, he was nominated for the CPL U21 Player of the Year Award, which was ultimately won by
Sean Rea. In November 2022, he signed a two-year extension with the club. In August 2023, De Rosario was disqualified from official competitions for one month by the Canadian Centre for Ethics in Sport, after he had tested positive for
THC in an in-competition
urine test; in an official statement, the player confirmed he would abide by the sanction, while stating his ingestion of THC was accidental. On October 7, he scored a brace in a 2–1
league victory over
Vancouver FC, which allowed York United to qualify for the play-offs. At the time of his departure from the club after the 2023, he was the club's all-time leading goalscorer with 19 goals. In March 2024, he moved to the
Tacoma Defiance in
MLS Next Pro. He scored his first goal on March 31 against
LA Galaxy II. On May 9, 2024, he joined the
Seattle Sounders FC first team on a short-term loan, making his debut in a
2024 US Open Cup match against
Louisville City FC. He signed another short-term loan later that month as well. He earned MLS Next Pro Player of the Month honours for April 2025. On May 3, 2025, after signing another short-term loan, he made his
Major League Soccer debut for the Sounders against
St. Louis City SC. On 8 May 2025, Rosario signed a first team contract with the Sounders, joining for the remainder of the
2025 MLS season, with club options for an additional two years. ==International career==