The injury occurred 40 minutes into the match when Reading's Jay Tabb was fouled by Jay Rodriguez and Tabb tumbled into Lallana's left leg.
"No comment" from #SaintsFC on Adam Lallana's knee injury. Suggests to me that it is pretty serious.
— Simon Peach (@SimonPeach) December 11, 2012
While Southampton have had their Saturday match with Chelsea postponed due to Chelsea's participation in the FIFA Club World Cup, a three-week absence would mean Lallana would miss at least the next four Premier League matches, plus the FA Cup game against Chelsea.
Missing Lallana will create two voids that will need to be filled.
First, the role as Southampton's on-field captain will need to be resolved. Kelvin Davis is the current club captain, but there is no guarantee that Davis will be on the pitch, as Paulo Gazzaniga may be back from his shoulder injury by then. The other choice would likely be Jose Fonte, who took the captain's armband from Lallana against Reading.
Second is what Manager Nigel Adkins will do tactically with Lallana's left wing position.
One option appears to have Southampton switch into a more-conventional 4-4-2 with Gaston Ramirez going out to the wing and Jay Rodriguez playing as a second striker next to Rickie Lambert.
The other option would be to maintain Adkins' preferred 4-3-3/4-5-1 hybrid and keep Ramirez in a central role. This would most likely mean Steven Davis, Emmanuel Mayuka or Guly Do Prado playing in a wing role, with Jason Puncheon switching to the left side if need be.
Another option would be to move Danny Fox or Luke Shaw up to left wing from left back and use the other in the left back role, or shifting Maya Yoshida out to left back with Jos Hooiveld coming into the lineup.
The January calendar isn't so kind, as Southampton will have to face Arsenal, Everton and Manchester United, plus most likely the make-up fixture against Chelsea in Premier League play and the FA Cup match against Chelsea.