Pantani had to abandon the 1993 Giro d'Italia with tendinitis after Carrera raced him too hard as a stagaire in 1992 and a domestique in Mexico & Trentino earlier in spring! If my information is correct he didn't race again in 1993 as well. Of course he would be coming back stronger the following year, winning back-to-back mountain stages and finishing 2nd at the 1994 Giro d'Italia, respectively finishing 3rd & winning the youth classification against Virenque at the 1994 Tour de France!
So Dumoulin not racing again at all this season might be not as dramatic as it sounds right now! The problem of course is that Dumoulins tendon is harmed by a fall and a subsequent injury, while Pantani's was simply stressed too much. I've got some worries, too. Then again, apparently Jumbo is confident that Dumoulin is 100% back in action in 2020!
As he wants to leave anyway, maybe Dumoulin simply doesn't have any desire to race again and potentially once again stress his knee too early this year anymore. Simply healing out completely and focus on 2020 with Jumbo already! So Ruben's suggestion sounds quite plausible to me, given his latest injury record and the ongoing contract negotiations!