While I really don’t think that Man of Steel will be doing horrible in the box office, it now becomes important that it does well. If it doesn’t do well for some horrible reason, the Justice League movie might not be made.
To summarize what the Warner Bros. movie division president Jeff Robinov said in an interview with Variety, the Justice League movie is dependent on how well the Superman reboot does. Besides the recent Nolan Batman movies, Warner Bros. has not done much with its access to the DC library. And since Green Lantern (2011) did not do so well, they might be wary about starting a huge project like Justice League that is slated for around 2015.
Like I said earlier, I highly doubt that Man of Steel will do badly. But with another entire project on the line, here’s to hoping that it will do well!