maybe he's been told by the manager that he's off too so he's jumping ship first...
Even if he has 5/6 years of an extra £100 a wekk, that will more than make up for the 2-4 European games a season he'd play with Rovers (If he was only interested in european game he wouldn't have left Linfield...). Most players born in the ROI have no emotional attachment to the League, and would jump ship to the UK for £50 so it can't be a surprise when a NI based player does it.
I think the only move thats ever really shocked me (in terms of thinking the player was above the move) was Hoolahan to Livingstone but that worked out for him too. maybe mannus is lookinga t the a similar route