Complex sentence: They subsequently transferred him to the Levski football academy, where in the following years Nedkov established himself as a key player for the youth team, scoring 90 goals in 142 games.
Split into simpler sentences: They subsequently transferred him to the Levski football academy. In the following years, Nedkov established himself as a key player for Levski's youth team, scoring a total of 90 goals in 142 games.