I think that for him it’s important to win the first games not to make a good impression, but to stay at Real. If it becomes clear from the first rounds that Real under Alonso is not able to win, they will quickly find a replacement for him. Alonso got the coaching position at Real partly because of the trust in him, but that only worked at the beginning; later he will need to confirm it with victories. And if that doesn’t happen, and Barcelona is already ahead of them at the start of the season, Perez will look for a replacement he always acts quickly.
Xabi Alonso has to realize, Ancelotti managed this team for 4 seasons, won 2 la ligas, won 2 UCL's, and he was still let go. So it's really not that easy to stay, plus just wins isn't enough, consistency is important. If Ancelotti won those back to back, like 1 la liga, then next season no la liga but ucl, and then back to just la liga, then back to just ucl, he would have stayed. But he had 2 great years, and 2 horrible years. Because of that he was let go.
Xabi needs consistency, even if he fails to win one thing ,he has to win another, he has to have "some" achievement at the end of the season to say he did alright. So either win la liga or win ucl and then he will keep his job, if he fails to win either, then he will be let go possibly.