There has been no timescale put on the Dutch goalkeeper's injury, so could he miss big games against the likes of Tottenham and Liverpool?
It heaps more misery on manager Alan Pardew who is under pressure to get results on Tyneside after a disappointing start to the season has seen them scrapping towards the bottom of the table, as the club have yet to register a Premier League win so far this season.
Krul has been amongst the best goalkeepers in the league for the past few seasons and his loss will be felt by Newcastle.
The true extent of his injury isn't yet known so no timescale has been put on a recovery with a club statement suggesting he will be assessed ahead of this weekend's game with Leicester City.
Ligament injuries tend to vary from a matter of a couple of weeks to several months on the sidelines, but the tone of the statement could suggest they feel the damage isn't so serious for the Dutch international.
If he misses the game against the Foxes, he could miss two huge games for the club with the trip to White Hart Lane to take on Tottenham Hotspur and the home clash with Liverpool at St James's Park a week later.
The last thing a Newcastle side need towards the bottom of the table is their keeper - who performed heroics at White Hart Lane last season in a 1-0 win - to be sidelined against sides that will look to take the game to them from the first whistle.
It doesn't spell good news for Alan Pardew, or the side in general. Newcastle fans will be praying their goalkeeper can return to fitness swiftly to give themselves the best possible chance of success through a difficult period of fixtures.