Each of the Premier League's 20 clubs have someone they probably cannot do without if objectives for the season are to be met.
For Everton, that's Idrissa Gueye. Seamus Coleman's dreadful injury and long layoff might prove costly to Ronald Koeman's Everton, but the loss of midfield engine Gueye, a gem of a signing from relegated Aston Villa, would probably be more harmful.
To see who is crucial to other Premier League clubs' success, click here.