Early years / Espanyol

Born in Sabadell, Barcelona, Catalonia, Tello started playing at the age of 11 with CF Can Rull, moving to FC Barcelona shortly after, and being loaned to another side in the region, CF Damm, for one year.

In 2008 his contract expired, and RCD Espanyol signed him; he made his senior debuts in the 2009–10 season, playing four games with the B side, with the club being relegated from the third division.

Barcelona

In June 2010 Tello moved back to FC Barcelona B, with the team in the second level. On 9 November 2011 he made his debut for Barça's first team, playing the full 90 minutes in a 1–0 away win against CE L'Hospitalet for the season's Spanish Cup; he scored a brace in the second leg, a 9–0 routing.

Tello made his first La Liga appearance on 28 January 2012, coming on as a substitute for Adriano in the 75th minute of a 0–0 away draw at Villarreal CF. The following weekend, in his first league start, he opened the score in the 8th minute of a 2–1 home win against Real Sociedad, after a through ball by Lionel Messi, and played the entire match.

On 7 March 2012, Tello replaced Andrés Iniesta early into the second half of a match against Bayer 04 Leverkusen at the Camp Nou, making his debut in the UEFA Champions League: within two minutes he scored his first goal, after Cesc Fàbregas played him through over the top and he ran from the half way 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).

Even though his future initially seemed uncertain, Tello managed to convince new manager Tito Vilanova and stayed in roster for 2012–13, inclusively going on to start in several games. On 19 September 2012 he scored the opener and assisted Messi in the second goal, in an eventual 3–2 home win over FC Spartak Moscow for the season's Champions League group stage, where he was also chosen as Man of the match.