You'd kind of hope that a big-name game releasing on 5 platforms for which (across the four for which install bases are known) there is a combined install base of over 175 million units would have a higher combined debut than a game releasing on just one platform with an install base just over 6 million. Especially when the former releases a few days earlier in the week, thus having more days to sell for.
Indeed, I'd suggest that, if Watch Dogs doesn't manage to outsell MK8 by a factor of 2 to 1 for the week across formats, then it has been outperformed easily by MK8.
Don't worry, it has done that. I'll take a stab in the dark here and guess that Watch Dogs PS4 will be the best selling game for the week.