Well considering how Sunweb was unable to give him proper mountain support for years I wouldn't be surprised if he would leave them. The handling of his knee injury obviously worsens the situation too. There are two main questions to ponder on:
1) How expensive breaking the contract is? What price has Doumolin have to pay should he leave now? He still has 2 or 3 years left of his contract.
2) Which teams needs him and could afford him (also considering the price mentioned in 1)?
As for 1 I have no idea but as far as I know Dumoulin's salary isn't that high (around 2 million €?) so the fee for breaking his contract is probably not that high either.
As for 2 I think Astana, Bahrain, INEOS, BORA, UAE, DD, Lotto, CCC and Katusha would consider signing him. From these teams I think Astana, Bahrain, INEOS, UAE and maybe Katusha could afford him. Astana has proper GC riders in Lopez and Fuglsang so I don't think they would offer a serious bid. So like it or not as I see it it's between Bahrain, INEOS and UAE.
As for INEOS they could definitely afford him but they don't really need him. However as Dumoulin is arguably the biggest threat to INEOS in the GCs they might sign him just to strengthen their position immensely. On the other hand Tom wouldn't go to INEOS in my opinion as he values success higher than money and with INEOS it's highly probable that he wouldn't be able to ride for the GC in the TDF for the next 1-2 years despite that it should be his main target from now on.
So I think Bahrain is the most likely option besides staying at Sunweb for one more year.