What guy left? Where'd you read that? It's not uncommon for devs to come and go throughout development, especially depending on their role. If there was any info on who left or why it may be easier to deduce whether or not it could mean trouble for the project, but at this stage, if it were say someone working on the narrative of the game I'd say it's likely not a concern as it could just be his work is done.
edit: Are you talking about
this guy? Don't know for sure exactly what his role was or what part of the game he was working on as he's based out of Ubi Montreal and Ubi Toronto are leading FC6 development, so I'd say this is less distressing news than had it been a senior dev from Toronto. Ubi has been chopping up development of it's games for a long time now, but we don't know what aspect of FC6 Montreal is working on or how much impact this particular designer had on the game as a whole, so it's hard to say I suppose.