Here's what the wiki has (from the Terry McLaurin page):
The injury: In the first quarter of Monday Night Football vs. the 49ers on Oct 19, McLaurin suffered a severe left knee injury — his knee buckled on a non-contact play while trying to get free from CB Deommodore Lenoir on a third-and-9. He was carted off with an air cast and ruled out for the rest of the game (he'd had the Commanders' biggest play of the night, a 27-yard catch, moments earlier).
How long is he out? The exact diagnosis and severity were still unclear per the latest report, but the cartage and immediate ruling out strongly suggest something serious — the page says he'll "likely be sidelined for multiple weeks" at minimum. No specific return timeline has been set yet.
Fantasy context: This is a brutal spot for the Commanders — Jayden Daniels is already out with blood clots and Marcus Mariota is starting. If you roster McLaurin, you're likely without your WR1 for a while, and Stefon Diggs becomes the clear top target in Washington while he's out. (from the Terry McLaurin and Washington Commanders pages)