The Leicester Tigers scrum-half was taken off early on in Rome on Sunday, his evident pain souring what proved to be a commanding 46-15 win for England.
And on Monday the Rugby Football Union (RFU) confirmed the 28-year-old had ruptured the medial collateral ligament in his left knee, bringing an end to his tournament.
Youngs will undergo a further assessment with a surgeon later this week, but the RFU said he could be sidelined for up to four months.
Earlier on Monday, coach Eddie Jones called up Saracens' Richard Wigglesworth as a replacement, with England facing Wales at Twickenham on Saturday.
The news is also a huge blow to the Tigers, who are struggling in the bottom half of the Premiership.
| Injury Update - Ben Youngs
Ben has been ruled out for the remainder of Six Nations campaign.
Scans have shown that Ben ruptured his MCL in his left knee and is having further assessment with a surgeon this week.
At this stage, he is likely to be out for four months. pic.twitter.com/HVBPu2BgsP
— Leicester Tigers (@LeicesterTigers) February 5, 2018
Photo Getty Images. Caption: England scrum-half Ben Youngs