Sources emphasized that there's no drama behind the casting change. MacFarlane brought Seyfried on board as part of a "creative decision" related to plot.
There's no mention of Seyfried's role in the film, so it's unclear whether she'll play a love interest opposite Wahlberg or someone else.
Ted 2 is slated for a summer 2015 release, but MacFarlane's latest film, A Million Ways to Die in the West, also stars Seyfried and comes out this May.
Notable deaths of 2014 [PHOTOS]
Jimmy Kimmel expecting third child