Complex sentence: Ronaldo became Manchester United's first-ever Portuguese player when he signed for £12.24 million before the 2003--04 season, which, at the time, made him the most expensive teenager in English football history.
Split into simpler sentences: Ronaldo became Manchester United's first-ever Portuguese player when he signed before the 2003--04 season. His transfer fee of £12.24 million made him, at the time, the most expensive teenager in English football history.