It is alleged that Alsono committed tax fraud back in 2015, and there is a chance he could get a big sentence if he is found guilty.
He is also facing a fine of up to £8 million. The claims originate from El Confidencial in Spain.
If previous tax cases involving high profile athletes in Spain is anything to go by – Alonso will not face actual time in jail, but may get a suspended sentence and a massive fine.
Alonso had a glittering career at club and international level, winning the World Cup, European Championship (twice), UEFA Champions League (twice), La Liga, Bundesliga (three times) and domestic cup competitions in England, Spain and Germany.