He was still only 18 years old when he went on to make three appearances for Dinamo Zagreb in the UEFA Champions League group stage in the autumn of 1998, starting in two matches and becoming notable for scoring the club's first goal in the UEFA Champions League when he opened the scoring in the 7th minute of Dinamo's fourth group match, against Porto at home in Zagreb. Dinamo won the match 3–1 and went on to win 1–0 against Ajax Amsterdam away and draw 1–1 against Olympiacos at home, but failed to advance to the quarterfinals after finishing second in their group. Mikić appeared in all of Dinamo's three home matches, playing as a striker.