Barca underestimated Granada and that was why the two goals was scored in the first half. Some of Barca key players were not in the match and this made it more difficult for them to play and secure three points. Barca scored a late goal which was suppose to give them a win but it was ruled out because Ferran Torres was on offside. It is better that they could draw the game than losing it. Currently, Real Madrid is three points ahead of Barca and this might be a big point, if Real Madrid fails to lose any match. Barca needs more consistency in their performance.
I don't think it's possible for Xavi and his players to underestimate Granada, in the sense that La Liga is very competitive. Losing points or drawing will make ranking in the standings stagnate. Barcelona draw was not only influenced by the absence of several key players, Xavi squad was overwhelmed in facing Granada style of play which implemented a 5-4-1 formation. This means that Barcelona appeared to attack to penetrate Granada defense, and dominated possession of the ball by more than 80%, Granada only took advantage of counter attacks and proved effective in scoring quick goals.
In my opinion three points is not too big, even though we know that Madrid and Barcelona will be determined to get full points in their next match. La Liga has only been running for nine weeks, meaning there are still 29 matches left to reach the end of the season. Many things can happen, it is very likely that Madrid and Barcelona will take turns leading the standings, don't forget that Girona and ATM could be surprising dark horses.