Complex sentence: Haaland finally retired in July 2003 after failing to recover full fitness from an injury to that same knee the foul from Keane Håland believed to be the root cause as after the foul from Keane he never played a full match again.
Split into simpler sentences: Haaland finally retired in July 2003 after failing to recover full fitness from an injury to that same knee the foul from Keane. However Håland believed the foul from Keane to be the root cause of the later injury as after the attack from Keane he never played a full match again.