The article is talking about his health, having 3 serious injuries in his knee and one in his back in the past 4 years, all required surgeries. It didn't mention a GK we want to sign as a source, but rather as in "other outlets has mentioned those GK".
The journalist is a one that follows Barca, not a madritista.
Doing some search, his last injury can be career altering one, it cause stiffness in movement in many cases. It is the same injury that Moussa Wague had, and the one that ended Ronaldo Nazario career in Europe. Very small sample size as it us rare one in football.
Ter Stegen may never be the same again.
PS: The club didn't make him captain, he was voted as one 3 years ago when Pique retired.