Knee issues (am unfortunately familiar with the process having had three ops on my ACL) are tricky. Aged 20 you'd have to think that any chronic ailments such as patellar tendinitis are not an issue, so we come down to injuries to the knee.
Firstly, what is injured? The serious issues, that is ruptures of all kinds, (serious) tears of all kinds you can probably rule out, especially as Lego played on the weekend. Nobody, but nobody would do that, whether or not they were angling for a move away. Nor would the club let them. Nor could the player in question play to any level (I stupidly played cricket the weekend after rupturing my ACL and, trust me, you just can't play sport in any capacity). So we come to the most likely scenario, which is meniscus essentially, slight tears to the ACL, MCL injuries and so on. In other words, injuries that need treatment but are risky when it comes to splashing out a million on a player who won't even likely be first choice for them.
So then how to treat it? The good news is that a straight arthroscopic intervention followed by, for example, meniscus needling or cutting the affected bit will see Lego out of action for 6 to 8 weeks or so. Similar periods of recovery if you treat it conservatively with brace and rest.
What I'm saying is, given the situation something like "Preston pull out, Lego out till new year" is extremely unlikely. Lego may need an operation which is never ideal, but such things go under "minor surgery" nowadays, recovery times at that age are faster and he should come out of it the same player as before...and all roughly in time for pre-season training.
That is my take on it anyway...