On 20 December 2017, Frattesi made his professional debut in a 2–1 away defeat against Serie A club Atalanta in the round of 16 of Coppa Italia, he was replaced by Matteo Politano in the 74th minute.[3] In that season he made also several bench appearances for Sassuolo's senior team, but he did not appear on the field.
Loan to Ascoli
On 16 August 2018, Frattesi joined to Serie B side Ascoli on loan until 30 June 2019.[4] Ten days later, on 26 August, he made his Serie B debut for Ascoli as a substitute replacing Tomasz Kupisz in the 57th minute of a 1–1 home draw against Cosenza.[5] One week later, on 2 September, Frattesi played his first match as a starter, a 2–0 away defeat against Perugia, he was replaced after 61 minutes by Enrico Baldini.[6] On 15 September he played his first entire match for Ascoli, a 1–0 home win over Lecce.[7] Frattesi ended his season-long loan to Ascoli with 33 appearances, including 26 of them as a starter, and 2 assists.[8]
Loan to Empoli
On 15 July 2019, Frattesi was signed by Serie B club Empoli on a season-long loan deal.[9] On 11 August he made his debut for the club as a substitute replacing Filippo Bandinelli in the 46th minute of a 2–1 home win against Reggina in the second round of Coppa Italia.[10] On 25 August he made his league debut for Empoli as a substitute replacing Karim Laribi in the 72nd minute of a 2–1 home win over Juve Stabia.[11] On 21 September, he played his first match as a starter for the club, a 1–0 home win over Cittadella and three days later his first entire match, a 3–2 away win over Pisa, where he also scored his first professional goal and the winning goal in the 95th minute.[12][13] Frattesi ended his season-long loan to Empoli with 41 appearances, 5 goals and 4 assists.[14]
Loan to Monza
On 16 September 2020, Frattesi joined newly-promoted Serie B side Monza on a one-year loan.[15] On 25 September he made his league debut for the club as a substitute replacing Marco Armellino for the last 17 minutes of a 0–0 home draw against SPAL.[16] On 20 October he played his first match as a starter for the club in Serie B, where he scored his first goal for Monza in the 46th minute of a 1–1 away draw against Pisa.[17] Four days later, on 24 October, Frattesi played his first entire match, a 2–1 home defeat against Chievo.[18]
On 1 May 2021, Frattesi scored his eighth goal for Monza against Salernitana;[19] winning a personal bet he made with coach Cristian Brocchi.[20] Frattesi was nominated in the Serie B Team of the Season, finishing the season with eight goals and two assists.[21]
Loan to Inter Milan
On 7 July 2023, Frattesi joined Inter Milan on a season-long loan with an obligation to buy.[22][23] On 16 September 2023, Frattesi scored his debut goal for Inter Milan in a 5–1 win over AC Milan during the Derby della Madonnina at San Siro.[24][25] On 19 January 2024, Frattesti scored in a 3–0 win over Lazio in the 2023 Supercoppa Italiana semi-finals.[26] Inter went on to win the title, defeating Napoli 1–0 in the final on 22 January, with Frattesi making a second-half substitute appearance.[27][28] On 8 April, he scored a decisive stoppage-time goal to help Inter to a 2–1 win over Udinese in the league.[29] Inter finished the 2023–24 season as Serie A champions, after defeating cross-city rivals Milan 2–1 on 22 April; the match culminated in a brawl, during which Milan defender Davide Calabria was sent off for striking Frattesi.[30][31]
With the Italy U19 he took part in the 2018 UEFA European Under-19 Championship, and scored one goal in the tournament, which came in a 3–2 win against Portugal in the first round;[34] Italy reached the final, where they faced the same opponent, losing 4–3 after extra time.[35]
The following year he took part in the 2019 FIFA U-20 World Cup with the Italy U20 squad. He scored in Italy's opening 2–1 win against Mexico,[36] and in a 4–2 win over Mali in the quarter-finals;[37] the team were eliminated in the semi-finals by Ukraine following a 1–0 defeat,[38] eventually finishing the tournament in fourth place after losing out 1–0 to Ecuador in the bronze medal match.[39]
He made his debut with the Italy U21 squad on 6 September 2019, scoring two goals as a substitute in a 4–0 friendly win 4–0 against Moldova in Catania.[40] He was included in the squad for the 2021 UEFA European Under-21 Championship,[41] where the team reached the quarter-finals, losing out to eventual runners-up Portugal 5–3 in extra time.[42]
Considered to be one of the most promising young Italian talents of his generation, Frattesi is a dynamic, technically gifted, and physical midfielder, who is known for his stamina, tactical qualities, and ability to assist his team both offensively and defensively. He is capable of both creating attacking plays and scoring goals himself due to his ability to read the game and make late runs into the box. Although he played as an attacking midfielder in his youth, he normally plays as a central midfielder in the mezzala role, but is also capable of playing as a defensive midfielder.[50][51][52]