I think if it were a longer season with a longer training camp, Spooner might have a better shot at breaking camp with the team. With only 48 games, they might decide that there isn't enough time to try and break someone in and deal with the ups and downs that a true NHL rookie will go through. Not that Bourque is any type of established NHL player, but they might have a better idea of what they will get from him on a day to day basis, at least to start.
That being said, it wouldn't shock me to see Spooner be the guy from the get-go, I think the Bruins are really high on him as a player. Even if it is not at the beginning of the year, I would expect him to be called up and contribute to the team at some point.