I think it's too early to criticize him. He's still adjusting to Spain, and tactically, his team isn't doing badly. We criticize Real Madrid for scoring one goal, but isn't that how football is? Sometimes you score one goal and win the game. The important thing was to earn the three points, and they did. I think we need to be a little more patient with Xabi Alonso.
Real Madrid is a big team for fans not to criticize the manager when the performance of the team is not going well, though they played their first game league game of the season fans were supposed to understand that the manager may be considering the best formation to use in the team but if the team is not able to improve in their next games then Xabi Alonso should be ready to start answering questions cause he's handling the best and biggest team in the world.
It's true that Real Madrid should have been able to win the match easily, scoring more goals to assert their dominance. However, to achieve all of that, Xabi Alonso certainly needs a bit of time to adapt, just like other managers when they first arrive. In my opinion, Xabi Alonso is currently experimenting with his main squad, and it's highly likely that their performance will improve as the season progresses.
Real Madrid fans will certainly criticize Xabi Alonso if the expected improvement never happens, and I think he knows what's at stake. The victory over Osasuna was a great start, and it's highly likely that Real Madrid will also win their next match against Real Oviedo. Overall, Xabi Alonso seems capable of meeting expectations.