In 2019 he joined Portuguese club Benfica for a €20 million fee, returning to Spain's top flight with Espanyol six months later for the same price and signing for Rayo again in 2022.

He played his first senior match on 8 April 2012, featuring the last 21 minutes for the C team in a 2–0 home win against CF Pozuelo de Alarcón in the Tercera División.

[9] He made his competitive debut for them on 29 October of the same year, replacing Karim Benzema in a 4–1 away defeat of UE Cornellà in the round of 32 of the Copa del Rey.

[14] He was the second-highest scorer of the season with 24 goals, including hat-tricks in victories over Lorca FC, Cultural y Deportiva Leonesa and CF Reus Deportiu,[15][16][17] as the club from Vallecas won the league and were promoted back to the top flight; this haul was a season record for any player in its history,[18] and he was also named Segunda División Player of the Month in February and April 2018.

[25][26] He debuted for the club as a starter in the 5–0 thrashing of crosstown rivals Sporting CP in the Supertaça Cândido de Oliveira on 4 August.

[31] Three days after joining, he made his debut away to UD San Sebastián de los Reyes in the third round of the national cup, coming on in the 61st minute for Jonathan Calleri and wrapping up a 2–0 win.

[37] De Tomás scored in a 2–1 home win over Real Madrid on 3 October 2021 to start a run of goals in five consecutive matches,[38] though he was sent off for an aggression towards Athletic Bilbao's Yeray Álvarez in the fourth of them.

[39] Another series of goals over five games included a red card in a 4–1 loss to Real Betis at the RCDE Stadium on 21 January and was concluded with an equaliser in a 2–2 draw in the Derbi barceloní against FC Barcelona on 13 February.

[47] In November 2021, due to Ansu Fati's injury, De Tomás was called up for the first time to the senior Spain national team, ahead of 2022 FIFA World Cup qualifiers with Greece and Sweden.