De Rosario is a product of the Toronto FC and New York City FC academies.[2] He spent the 2020 preseason with the NYCFC first-team, appearing in four matches, scoring twice, including a goal against Brazilian club Palmeiras.[3][4] He departed NYCFC after 2020, and in early 2021 went on trial with Spanish clubs UD San Sebastián de los Reyes[5] and CD Lugo.[6][7]
In September 2021, after a successful trial, De Rosario signed a three-year contract with Ukrainian Premier League club Rukh Lviv,[10] 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.[11] In February 2022, he left the club by mutual consent,[12][13] nine days before the 2022 Russian invasion of Ukraine after United States President Joe Biden urged all American citizens to leave Ukraine.[14]
After leaving Ukraine, De Rosario joined Canadian Premier League club York United on trial during their pre-season in March 2022.[15] The next month on April 6, York announced they had signed De Rosario to a one-year contract, with options for 2023 and 2024.[16] He made his debut in York's season opener against the HFX Wanderers on April 7.[17] In De Rosario's next match on April 15 against FC Edmonton, he started his first match, in which he scored his first goal.[18][19] On August 2, he was named Player of the Week for the first time for Week 17.[20] In September, he became the club's all-time leading scorer, when he scored his 12th goal.[21] At the end of the season, he was nominated for the CPL U21 Player of the Year Award,[22][23] which was ultimately won by Sean Rea. In November 2022, he signed a two-year extension with the club.[24][25] 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.[26][27] 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.[28] At the time of his departure from the club after the 2023, he was the club's all-time leading goalscorer with 19 goals.[29][30]
De Rosario was eligible to represent the United States (his country of birth), Canada (through his parents) and Guyana (through his paternal grandparents).[36]
In November 2023, De Rosario accepted a call-up to the Guyana senior national team ahead of their CONCACAF Nations League B matches against the Bahamas and Antigua and Barbuda.[37][38] He made his debut and scored his first international goal for the Golden Jaguars on November 21, in a 6–0 win over Antigua and Barbuda.[39][40][41] In November 2024, he participated in a training camp with the Canada senior team as his father was serving as an assistant, before then re-joining the Guyana team for their CONCACAF Nations League matches.[42] On November 19, he scored a hat trick for Guyana in a 5-3 victory over Barbados in the CONCACAF Nations League.[43][44]
^"Де Росарио Осазе покинул Рух" [De Rosario Osaze left Rukh]. Football UA (in Ukrainian). 16 February 2022. Archived from the original on 16 February 2022. Retrieved 16 February 2022.
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