It's very difficult to predict the outcome of the match, but Barcelona's preparations have been excellent, so we can say they're fully prepared for La Liga. So, while it's difficult to predict the outcome, I'm betting Barcelona will win.
The only difficulty is Mallorca playing home with the advantage they can perform well to prove to their fans, will Barcelona lose to 3 points in the first lef of La Liga, despite home advantage i believe Barcelona is the favorite and will make use of the 3 points tomorrow.
Barcelona are not only winning, they make other team look really underperforming, always going over 2.5 goals in each game is another crazy part of playing opening game against Barcelona, even Mallorca will be extremely cautious knowing what their opponent can do if given the chance.
Barcelona to lose that game is not the most probable result, but I agree that Mallorca playing at home could cause Barcelona to have some problems during their first game. Nevertheless it is Barcelona and if you look at their team and if they are able to play a normal game, not a catastrophic game, I think they will win without too much hassle. The gap in the quality between those two teams is huge.