[2][3] Popescu was loaned out by FCSB to second tier side Turris Turnu Măgurele on 20 August 2020, but was immediately recalled to his parent club after general manager Mihai Stoica and owner Gigi Becali found out the deal was agreed over their heads.

[9][10] Popescu scored the only goal of a championship play-off match against CFR Cluj on 17 April, representing FCSB's first win at the Dr. Constantin Rădulescu Stadium in almost eight years.

[14] He scored his first goal in European competitions on 3 August, his late winner helping defeat CSKA 1948 3–2 at home and 4–2 on aggregate in the second qualifying round of the UEFA Europa Conference League.

[16][17] He made his debut for the under-21 side in March 2021,[18] aged 18, after entering as a substitute in the UEFA European Championship group stage matches against Hungary and Germany, which ended 2–1 and 0–0, respectively.

[22][2] Popescu's breakthrough in FCSB's senior team was highly regarded by Romanian pundits—Cornel Dinu described him as a "very talented kid of extraordinary quality" who "has similarities to Florea Dumitrache",[23] Gabi Balint stated he is a quick-thinking genius "just like [Gheorghe] Hagi was",[22] and Basarab Panduru emphasised his versatility.

[24] His former coach Daniel Pancu regarded him a "fantastic talent",[2] while Dorinel Munteanu said that he is a "youngster who looks different from what we have in Romania" and that from his "generation, from [Gheorghe] Hagi—not even from Adrian Mutu, but from Hagi" he has "never seen such a lightness to play in depth or to provide the final pass".

[25] When discussing his weaknesses, Balint noted in February 2021 that Popescu should become more consistent during the full 90 minutes and build muscle mass,[22] and the following year Ioan Sabău made a similar remark.