There is no version of this article where I tell you to bench Patrick Mahomes. He is starting Monday night, he was a full participant throughout camp, and by his own account he feels stronger than he ever has. Start him. The argument here is narrower and, I think, more useful: the market has priced this return as though the injury simply did not happen, and the calendar says otherwise.
The specifics matter. On December 14 of last season, against the Chargers, Mahomes tore both the ACL and the LCL in his left knee. That is not a clean isolated ACL. Multi-ligament injuries carry longer timelines and more variable outcomes, which is why the standard published range for return to sport sits between nine and twelve months. He had surgery in Dallas with Dr. Dan Cooper shortly after, and he was cleared to practice roughly seven months later.
Now count forward. Kansas City opens against Denver on September 14. December 14 to September 14 is nine months to the day. He is not returning comfortably inside the window; he is returning at the absolute floor of it, on a two-ligament reconstruction, for a Week 1 game against a division rival. He will be wearing both a compression sleeve and a brace on that leg. None of that is a red flag on its own. All of it together is a reason to expect the early-season version of this player to differ from the pre-injury one.
The specific fantasy consequence is mobility. Mahomes has always generated a meaningful slice of his value from plays that break down, where he escapes the pocket and creates something from nothing. That is the first thing to go when a quarterback is protecting a knee, consciously or otherwise, and it is the hardest thing to see in a practice report. Denver's interior pass rush is well built to test exactly that, and the receivers he would normally buy time for are compromised. Rashee Rice is coming off knee surgery that cost him camp reps.
So start him, and adjust the expectation rather than the lineup. If someone in your league wants to trade for Mahomes at his pre-injury price this month, that is a deal worth listening to. The player will very likely be fine by November. The question is only what the first month looks like, and nine months after a torn ACL and LCL is not the moment to assume the answer is nothing.