Idispensible means that the player can't be missed, those players are the spine of your team. Important first team players are they players that will deserve a first team place in most matches.
For example: Ronaldinho is indispensible at Barca, Deco is a important first team player. Both play almost every match, but Ronaldinho offcourse is a more important then just someone that deserves a first team place | 
