Difference between Lorenzo being signed to Yamaha vs. Rossi being signed now to make the team stronger is that in the Lorenzo case it was done in search for progression and talent development for a future champ.
The champ was made, so Rossi had reason to not want Lorenzo on Yamaha. Scared of the futures?? Lorenzo is now a stronger rider than his ex teammate.
If Lorenzo is annoyed, which I suspect he should be, I don´t think it´s because he sees Rossi as a real sporting threat, more of a pain in the ... constant comparo and scoutiny circus for the next year.