The Holland-born goalkeeper has made 185 appearances for the first team during his time at Newcastle after joining the under-18 squad in 2006, going on to make his debut in August 2009.
Krul has failed to make a single appearance at Ajax, where he has been on loan, following a serious knee injury he sustained in 2015. The 28-year-old’s stay at the Dutch side is due to be cut short due to a lack of appearances.
The Hornets want a rival for Heurelho Gomes, who is the current back-up goalkeeper, as, during Sunday’s FA Cup defeat to Millwall, first-choice goalkeeper Costel Pantilimon picked up an injury.
Walter Mazzarri was expected to hold back on his search for another goalkeeper until the summer but with the aforementioned injury to Pantilimon, his hand could be forced to make a late move in the January window.