[5] His competitive debut came against Botev Plovdiv in the opening game of the Bulgarian League season on 18 July, and scored his first goal for the club in a 1–1 away draw.

[7] On 22 June 2017, Kostadinov signed as a free agent for Israeli Premier League club Maccabi Haifa on a two-year contract.

[8] He made his competitive debut for the club on 29 July, playing full 90 minutes of a 2017–18 Toto Cup Al match against Bnei Sakhnin at Acre Municipal Stadium; Maccabi won 4–1.

[10] On 1 July 2018, Kostadinov joined Russian Premier League club Arsenal Tula on a 2+1-year contract, for an undisclosed fee reported to be €200,000.

[11] On 29 July, he made his competitive debut in their season opener against Dynamo Moscow by replacing Kantemir Berkhamov for the final 32 minutes of a 0–0 home draw.

[13] Kostadinov earned his first cap on 7 October under new manager Petar Hubchev, playing the full 90 minutes in the 1–4 away loss against France in another qualification match for the 2018 World Cup.