The German international picked up the injury during Schalke's 1-0 win over SC Paderborn a week ago and had already been ruled out of Saturday's final league clash of the season at Hamburger SV, but tests carried out by national team doctor Hans-Wilhelm Müller-Wohlfahrt have revealed more damage than initially feared.
In addition to partially rupturing his syndesmosis ligament, the 27-year-old has also sustained a capsular injury and a bone oedema. As a result, he will have to wear a cast for the next fortnight before further examinations will plot a roadmap for recovery.
Nevertheless, it is not expected he will be fit in time for the start of pre-season training in July. After missing much of pre-season last year due to Germany's World Cup win, it will mean the second successive year that Höwedes misses part of the campaign's preparations.