Nzonzi was given the starting defensive midfielder role for the 2008–09 season and appeared in 36 total matches. He also scored his first professional goal in a 2–1 defeat to Strasbourg on 8 May 2009. The defeat to Strasbourg led to Amiens going winless in their final four matches, which led to the club suffering relegation to the Championnat National, France's third division of football. Despite the relegation, Nzonzi was applauded for his play and he was linked with a move to the English Premier League, with Blackburn Rovers and Portsmouth showing a serious interest in the young Frenchman, who many hailed as the next Patrick Vieira.
On 28 June 2009, it was reported that Blackburn Rovers had agreed a deal with Amiens to sign Nzonzi. The move was completed on 30 June, signing for a fee of £650,000, on a four-year contract.
On 15 August 2009, Nzonzi made his debut for Blackburn where he made his first start in a 2–0 loss against Manchester City and since making his debut, found himself very much part of Sam Allardyce's plans. He scored his first goal for Rovers on 4 October 2009 against Arsenal in the 6–2 defeat. After scoring his first goal, Nzonzi admitted he had been surprised at how quickly he had established himself in the side but was determined to keep his place in the starting XI. His second goal for the club was a 30-yard strike against Everton on 17 April 2010 in a 3–2 defeat at Ewood Park. On 3 May 2010, during the pre-match build up to the Arsenal game, he was announced as the Blackburn Rovers Player of the Year for the 2009–10 season, chosen by the fans.
Nzonzi earned his first call up to the France U-21 team by coach Erick Mombaerts on 1 October 2009 for their European Under-21 Championship qualification matches against Malta and Belgium. He made his debut eight days later in the first match, appearing as a 58th-minute substitute for Younousse Sankharé in a 2–0 victory. He earned his first under-21 international start in the match against Belgium, which ended 0–0.