[3] On 9 November 2011 he made his debut for Barça's main squad, playing the full 90 minutes in a 1–0 away win against CE L'Hospitalet in the round of 32 of the Copa del Rey;[4] he scored a brace in the second leg, a 9–0 demolition.

[7] On 7 March 2012, Tello replaced Andrés Iniesta early into the second half of a match against Bayer 04 Leverkusen at the Camp Nou, taking his bow in the UEFA Champions League: he scored his first goal within two minutes, after Cesc Fàbregas played him through over the top and he ran from the halfway line to net from just inside the penalty area for the 4–0; six minutes later, from a similar position, he made it 6–0 for the hosts in an eventual 7–1 win (10–2 on aggregate).

On 19 September 2012 he scored the opener and assisted Messi in the second goal, in a 3–2 home defeat of FC Spartak Moscow in the Champions League group stage, where he was also chosen as Player of the match.

[14][15] He scored his first goal of the season on 11 December in a 6–1 home rout of Celtic in the Champions League group phase,[16] and six days later he also found the net, in a 3–0 victory against FC Cartagena in the domestic cup (7–1 on aggregate).

[19] He scored his first goal for the club on 25 November, entering as a substitute for Ricardo Quaresma and concluding a 3–0 win at FC BATE Borisov in the group stage of the Champions League.

[23] He made his Serie A debut on 3 February, starting a 2–1 defeat of Carpi FC 1909 at the Stadio Artemio Franchi, and scored his first goal 18 days later in a 3–2 victory at Atalanta BC.

[26] He made his debut from the bench on 25 August in a 2–1 home win over RC Celta de Vigo,[27] and scored his first goal on 15 October as a consolation in a 6–3 loss to Valencia CF also at the Estadio Benito Villamarín.

[30] In 2019–20 he rarely started under manager Rubi and was linked to January moves to Espanyol or Celta;[31] he stayed and scored the late winner against Real Madrid on 8 March.

[33] On 22 May 2021, as the campaign ended with qualification for the UEFA Europa League via a 3–2 comeback win at Celta, he came on as a half-time substitute and received the first sending-off of his career for two yellow cards in as many minutes.