On 27 August 2008, Mihaylov made his debut for FC Twente in their 4–0 defeat to Arsenal at the Emirates Stadium in the second leg of the third qualifying round for the UEFA Champions League and went on to make his Eredivisie debut in a 1–1 draw at Roda on 30 August 2008. On 14 March 2009, Mihaylov made his second appearance for Twente in the Dutch league against Willem II, playing the full 90 minutes and earning himself a booking, managing to keep a clean sheet in Twente's 2–0 away win. On 29 July 2009, Mihaylov came on as a substitute for midfielder Kenneth Perez, following the dismissal of starting keeper Sander Boschker in the 27th minute of Twente's away game against Sporting CP in the first leg of the UEFA Champions League third qualifying round. He stopped the penalty taken by João Moutinho and made a number of key saves to keep the scores level at 0–0. Mihaylov played the full 90 minutes in the return leg and was not at fault for the Portuguese side's last-minute equalizing goal, which allowed Sporting CP to advance on the away goals rule.