The 26-year-old needs surgery after suffering posterior cruciate ligament damage during Sunday's 27-17 European Champions Cup win over Racing 92.
Tigers are already without Manu Tuilagi, although the England centre is near to a return after a groin injury.
"Not having Matt and Manu is clearly disappointing but we have good young players," said boss Richard Cockerill.
"We know the quality of Matt and Manu. But Manu is making good progress and we will hopefully have him back on the field soon.
"We have some options in the midfield with Matt Smith, Peter Betham and Jack Roberts, and Owen Williams has played there, so we will get on with coaching what we have got."
Toomua joined the club from Super Rugby side Brumbies before the start of the season, but Tigers' big-name signing was not fit until earlier this month.
His appearance against Racing 92 was just his second since arriving.
Toomua, who was part of the Wallabies squad that reached the 2015 World Cup final, made his much-anticipated debut for Leicester in the European fixture against Glasgow on 14 October.
Tigers lost the game 42-13 and Toomua admitted he was lucky not to be sent off for a tip-tackle in the early stages of the match in Scotland.
Leicester and Australia centre Matt Toomua faces six months out with a knee injury, reports BBC Radio Leicester.