Promoted to the first team by head coach Miguel Ángel Brindisi, Valeri made his Primera División debut in September 2003 against Vélez Sarsfield.

In his debut season, Valeri helped the club top the Western Conference standings, earning him the MLS Newcomer of the Year award.

In August 2020, Valeri helped captain the Timbers to the final of the MLS is Back Tournament, defeating Orlando City SC.

Born in the industrial city of Lanús, a suburb of Buenos Aires[2] on 1 May 1986, Valeri is the son of Luis, a former footballer turned shoe factory owner, and Monica, a former tennis player for the local athletic club.

[5] On 17 July 2009, Valeri joined Portuguese Primeira Liga club FC Porto on a two-season loan with a purchase option.

[7] Valeri then made his UEFA Champions League debut on 30 September 2009 during a 2–0 victory against Atlético Madrid, coming on as a stoppage time substitute.

[5] Prior to moving to the United States, two other Major League Soccer clubs were in discussions to sign Valeri – the Chicago Fire and Timbers rival Seattle Sounders FC.

[17] On 6 August 2013, after he scored four goals and earned eight assists in 20 matches, the Portland Timbers exercised their purchase agreement with Lanús and signed Valeri to a multi-year contract.

[18] A day later, on 7 August, Valeri scored a consolation goal for the Timbers in their 2–1 defeat against Real Salt Lake in the U.S. Open Cup.

[21] During the match on 6 August, Valeri providing the game-winning assist for Landon Donovan in the 70th minute to help the MLS All-Stars win 2–1.

During the Portland Timbers final regular-season match of 2014 against FC Dallas, Valeri tore his ACL and was sidelined to start the 2015 season.

[27] He opened the scoring for the club after just 27 seconds, deflecting a clearance from Columbus goalkeeper Steve Clark to give the Timbers the very early lead.

[28] Despite the losses of forward Maximiliano Urruti, defender Jorge Villafaña, and captain Will Johnson, the Timbers retained main offensive starters Valeri, Darlington Nagbe, and club top-scorer Fanendo Adi.

[28] There were also higher expectations for forward Lucas Melano, who arrived midway through the previous season as a designated player from Valeri's former club Lanús.

[31] A 4–1 defeat to the Vancouver Whitecaps FC on Decision Day meant that the Timbers finished two points behind 6th-placed Real Salt Lake in the standings.

[37] A little more than a week later, on 23 August, Valeri became the Timbers all-time top goalscorer; scoring the first goal in a 2–1 victory over the Colorado Rapids and breaking the record set earlier that season by teammate Fanendo Adi.

[38] Valeri's performances throughout the season were instrumental in securing Portland Timbers the top spot in the Western Conference standings and qualifying for the playoffs.

[41] That season, Valeri, alongside former Lanús teammate Sebastián Blanco, helped lead the Portland Timbers back into the MLS Cup playoffs.

[52] Due to the COVID-19 pandemic, the Portland Timbers participated in the MLS is Back Tournament held in Orlando, Florida, with Valeri acting as club captain.

[53] On 2 September 2020, Valeri became the third player in Major League Soccer history to score at least 80 goals and earn 80 assists – alongside Landon Donovan and Jaime Moreno.

[56] On 16 March 2011, Valeri made his international debut for Argentina at the Estadio San Juan del Bicentenario in a friendly against Venezuela.

[63] "He will be one of the better playmakers in the league, but more importantly Diego fits our identity and his play should boost production to help the club win games," stated Porter after Valeri signed with the Timbers.

[63] During his first season in Portland, Valeri adapted into the middle of Porter's 4–3–3 formation, forming a partnership with central midfielders Diego Chará and Will Johnson.

[65] That partnership allowed Valeri the freedom he needed to score ten goals and provide thirteen assists in his first season with the club.

[66] Considered to be a skilled passer who was able to get the ball to his forwards in great positions,[67] Valeri was not only known for his offensive ability but also for being able to contribute defensively by dropping back and helping start a Timbers counterattack.

[68] Prior to a match in 2014, former Toronto FC coach Greg Vanney stated that Valeri was "clever" and that "you don't really want to send your marking guy to the outside to deal with him".

[70] According to Timbers general manager, Gavin Wilkinson, this partnership helped Valeri reach a career-high 21 goals during his MVP season.

[70] Despite not being a full-game starter for the Timbers in 2020, coach Giovanni Savarese stated, "his movements are very sharp, his quality is still intact and he's taking care of himself in the best possible way.

[4] He had only visited the United States once, in 2004, when he was part of the Lanús side that took on a Japanese club in a pre-season friendly at Dignity Health Sports Park in Los Angeles.

[3] He has also developed a deep relationship with the supporters' groups of the Portland Timbers, regularly participating in local football events and matches.