Headache? More like a disaster waiting to happen. Too many midfield players and the best teams in the world usually do not have such an overflow as it can disrupt team chemistry. Blind and Fellaini(two of our best or important players last season) will most likely become bit part players. Mata and one of Carrick or Schneiderlin aswell. The latter dependent on who is more in form and fit. The former, if he does not play on the right. Pereira development will go backward next season unless he is loaned out, so no need mentioning him anymore.
All the top European teams have an abundance of quality midfield players - look at the talent in Munich's midfield for starters.
Also:
Carrick - Getting on and will be expected to play more reservedly at his age
Fellaini - Let's be honest, never going to be a regular starter under LVG, nor do I think he'll care if he's playing a more marginal role, I get the impression he loves being here regardless.
Pereira - He's still a youngster
Blind - I'm sure he was signed for his versatility, not to be a regular starter in the midfield. Its a long season so he'll get plenty of minutes in defence and midfield.
That leaves, Mata, Schneiderlin, Herrera and Swine's tiger as our firm first choice midfielders. They'll all play plenty of games, likely starting most of them too.