Under FIFA rules, you cant sign a player if he has already transferred in that year. Liverpool tried to sign Mascherano as well but i doubt it'll bare any fruits.
Furthermore, Tevez's style is similar to Rooney. They like to drop back further into midfield and plough their way through the defence. The question is, do we really need another player? Wouldn't and out-and-out striker be better?
Mascherano has'nt play much to convince others that he's of premiership material. It'll most certainly be a gamble.
It's January Window and it's always good to play safe in this period and you wouldn't like your dressing room to be in disorder with more and more players being brought in.
But of cos, those are just my humble opinions.

Cheers.
C.K