During the early stages of the 2009–10 season, Alderweireld established himself in the first team and starting XI. He became first-choice centre back along with fellow Belgian Jan Vertonghen following the departure of former captain Thomas Vermaelen to Arsenal. New Ajax coach Martin Jol praised the two defenders and kept faith in the duo after a string of good performances. During the 3–0 win against Heracles on 4 September, he scored his first league goal for the club when he opened the scoring with a header from Luis Suárez's corner-kick. On 27 January, he scored a late equaliser in extra time during the KNVB Cup quarter-final against NEC; Siem de Jong scored five minutes before the final whistle to steal a 3–2 win for Ajax. At the end of the 2009–10 season, Alderweireld was named Ajax Talent of the Year. He assured himself of a place in the Eredivisie giants' history books on 3 November 2011, heading in Ajax's 100th UEFA Champions League goal in the 2–1 Group G defeat by Auxerre. In the 2010–11 season, he scored a stunning 25-yard strike at the San Siro against Champions League group rivals Milan in a 2–0 away win. He followed this spectacular goal with an even more impressive 32-yard strike against Feyenoord in a 2–0 home win. In the 2011–12 season, Alderweireld made significant strides in his ability as a footballer, also forming a formidable partnership in the heart of the Ajax defensive with captain and fellow Belgian Jan Vertonghen. Alderweireld also showed some impressive offensive ability, scoring a remarkable goal against Heerenveen.