[3] He made his first-team debut on 27 July 2013, at 16 years 227 days, coming on as a substitute for Hristo Yanev in a 0–0 away league draw against Cherno More Varna.

[4] After the great debut season and a few trials in big European teams, Velkov got a serious injury, which took him out of football for a minimum of six months.

[8] Velkov scored his debut goal for Slavia in the league on 17 September 2016 in a match against Ludogorets Razgrad, lost by his team with 3–1.

[9] On 10 August 2018, Velkov signed for the Dutch Eerste Divisie team Den Bosch.

[20] Velkov earned his first cap on 26 February 2020, playing the first 75 minutes of the 1–0 home loss against Belarus in a friendly game.