The injury is serious enough to keep the international midfielder out for up to three months, as reported by Marca, meaning he misses out on much of the new era for the club under the guidance of Carlo Ancelotti.
While Alonso will always be missed because of his technical ability, his experience and his willingness to protect the defence with his positional work, Real would hope to have most major absences covered this season after making several signings in the transfer market.
In this instance, it will be new arrival Asier Illarramendi who is expected to step up and perform for the first team in Alonso's absence.
Illarramendi has struggled with fitness himself since joining Real, missing part of the preseason programme because of "feeling the strain both mentally and physically," something he has not fully recovered from.
He was not included in the opening squad of the season to face Real Betis, with Sami Khedira, Luka Modric and Isco operating in the midfield triangle in the 2-1 victory.
With Alonso out, Real are going to need a player in the centre who can not only take charge of dictating the tempo of the team—Modric is more than capable of that—but who also can afford extra protection in front of the defence.
Last term, Illarra ranked in the top 10 of the entire Spanish top flight for tackles made per game, significantly higher than any Real Madrid player, and had an 80.7 percent pass completion rate over the whole season—comparable with new team mate Khedira, who managed 81.7 percent success.
Illarra was also more often on the ball than the German, being one of the main focuses for Real Sociedad to play through in the centre and averaging more than 50 passes per game.
Both defensively and in a more creative role, then, Illarra has everything he needs to step into Xabi Alonso's not-quite-protective-enough boots and become the key player over the coming months that Real hoped he would when they shelled out £34 million for him.
Confidence and maintaining fitness himself will be the only barriers to the under-21 international quickly becoming a fixture in Carlo Ancelotti's new-look side.