Big deal. Probably tweaked the knee from the previous injury. Sprains can nag all year, as they are rarely given enough time to actually heal. There aren't enough weeks in the season. Just enough to dull the pain and get back out there. Not a major thing IMO.
I spent 2 seasons in a row in highschool with a nagging hip pointer. Actually, since summer was basically: camp, it's more like 3. I should have just taken 2 weeks off and let it heal. Instead, I kept playing on it. Finally, I ripped it out completely, including bone flaking off, and then was forced to lose a month.
There's something to be said for being "tough" and "playing through injuries". There's also something else called: being an idiot.
If he's going to re-injure his knee he needs to sit down. It's simply not worth the risk, for him or the team.