Complex sentence: He had been the assistant head coach at West Bromwich Albion, one of the teams he had served as a player; he was temporarily caretaker manager after Roberto Di Matteo was placed on gardening leave.
Split into simpler sentences: He had been the assistant head coach at West Bromwich Albion, one of the teams he had served as a player. He was also their caretaker manager after Roberto Di Matteo was placed on gardening leave.