וּמִכַּ֤ף מֶֽלֶךְ־אַשּׁוּר֙ אַצִּ֣ילְךָ֔ וְאֵ֖ת הָעִ֣יר הַזֹּ֑את וְגַנּוֹתִ֖י עַל־הָעִ֥יר הַזֹּֽאת׃
And I will deliver thee and this city out of the hand of the king of Assyria; and I will defend this city.
Rashi on Isaiah
And from the hand of the king of Assyria will I save you We deduce that he became ill before the downfall of Sennacherib.