Tach is a good role player: destroyer in the midfield. However, at the end of day, he is simply a destroyer who can only perform limited role.
Anyway here are other players who I feel does better job than what Tach specialize in:
Rino
Viera
Makelele
Davids
Keane
Gerrard
This is not to mention that there are many other players who can perform similar role to Tach: Gianichedda, Ambrosini, Jeremies, Zanetti, and co.
The matter of fact is there are many other players out there who can perform similar role hence he is far from being indispensible. On the other hand, there is no need to sell him for the sake of it.
What is clear, IMO, though is that as the way they stand I do not see Tachinardi as an ideal player for our team.
The major deference between this year and last year, I believe is that Tachinardi is missing Davids in the midfield.
Beside lacking passing ability, on other major limitation in Tach's game, IMO, is lack of atheleticism, an ability to cover grounds in the way Davids and Rino do. Nor does he have playing intelligence of some of the great registra in the game a la Redondo, Deschamp, and Albertini.
So as it stands we do not have a natural playmaker, nor dynamic midfielder such as Davids means that Tach ability to perform his role is limited and his weaknesses are exposed. Of course, if we add likes of Aimar or some other midfielder who can cover Tach weakness then I see a place for him in the team. However, I never seen a GM who signs player to simply utlilize his role player. At the end of day, I think future of Tach depends on the strategy of Moggi or Lippi or other coaches who may replace Lippi.
If he doesnt fit into our overall plan, then the wise thing is to sell him. There is nothing worse than disgruntled senior player in the team.