Tuilagi, who had just been recalled by England after persistent groin and hamstring injuries, went off after 10 minutes of the Tigers' 16-12 home defeat by Saracens.
"Unfortunately Manu sustained an ACL injury in the game on Sunday that's going to put him out for the rest of the season," Mauger told reporters.
"It's looking like a six-month recovery which is devastating for the bloke, he's worked so hard to get back to where he is."
The 25-year-old Tuilagi, who has 26 caps but only one under head coach Eddie Jones, had been due to join the England squad for a training camp this week.
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Instead, the Samoa-born player faces another lengthy period on the sidelines and is likely also to miss the British and Irish Lions tour of New Zealand in June and July.
He returned to the squad in March 2016 for the Six Nations, which saw England take a grand slam, after a 15-month layoff due to injury and disciplinary issues but the comeback was short-lived.
Tuilagi, who missed the 2015 World Cup, was forced to withdraw from England's triumphant 2016 tour of Australia due to a hamstring injury and missed the autumn internationals.
England, on an unbeaten run of 14 matches, start their Six Nations campaign against France at Twickenham on Feb. 4.
(Reporting by Alan Baldwin; Editing by Clare Fallon)
