Complex sentence: Playing in Yorkshire, he had attracted the attention of Herbert Chapman when the latter was manager of Huddersfield Town; when Chapman became Arsenal manager, needing a quality centre-forward, he signed Lambert for £2,000 in June 1926.
Split into simpler sentences: Playing in Yorkshire, he had attracted the attention of Herbert Chapman when the latter was manager of Huddersfield Town. As is so, when Chapman became Arsenal manager, needing a quality centre-forward, he signed Lambert for £2,000 in June 1926.