Complex sentence: Under the unwritten sportsmanship rule, Arsenal would expect the ball to be returned to them unhindered, but because this was not an injury, Sutton chased the ball instead of allowing it to be thrown back to Arsenal and won a corner from his efforts.
Split into simpler sentences: Under the unwritten sportsmanship rule, Arsenal would expect the ball to be returned to them unhindered. However, Sutton chased the ball instead of allowing it to be thrown back to Arsenal and won a corner from his efforts.