[2] Born in Klyuch, part of Petrich Municipality, Delev started to play football in Pirin.

After the union of the two Pirin clubs from Blagoevgrad in December 2008, Delev joined the part of the team, which plays in the top division.

On 4 March 2009, he scored a goal in a match against CSKA Sofia for the Bulgarian Cup and Pirin eliminated their opponent.

[10] Delev's debut came on 15 January, in a 0–2 home loss against Antalyaspor in a Süper Lig match, during which he appeared as a second-half substitute.

[11] On 24 January, Delev netted his first goal for the team, opening the scoring in a 1–2 home loss against Kayserispor.

[16] He made his Ekstraklasa debut on 16 July, replacing Mateusz Lewandowski in the 66th minute of Pogoń's 2–1 loss against Wisła Kraków.

He made his debut for the senior side on 26 March 2011, in the UEFA Euro 2012 qualifying match against Switzerland.

[21] His first Bulgaria goals came on his 14th appearance on 25 March 2017, scoring twice in the 2018 FIFA World Cup qualifying match against Netherlands, which ended in a 2–0 victory.