For the sake of argument, let’s assume MacReady is turned somewhere between getting back from the flying saucer and when Blair goes axe happy with the radio equipment.
I believe MacReady is a Thing here because I’m pretty sure he turns Blair. Again, a big deal is made about how everybody should prepare their own food and eat out of cans because the theory is that a small piece of the Thing can infect you. If you notice, when they subdue Blair and put him in the tool shed, MacReady waits for everybody else to leave and then he picks up a bottle of booze sitting in front of Blair and takes a big swig, and sets it right back down in front of him.
From this moment on, MacReady is suspicious as hell. Fuchs is the next to die and MacReady is the last person to see him alive. Notice when he enters the room to talk with Fuchs he makes sure to look up and down the hall to make sure they’re alone. Fuchs acts extremely suspicious of MacReady here, damn near looking like a deer frozen in headlights when he enters the room. He even has his hand on a glass jar (acid, maybe?) until he’s sure Mac isn’t attacking him.
MacReady leaves and the next thing you know the power goes out, Fuchs sees somebody and ends up outside, in the snow, where he finds MacReady’s torn-up clothes. Next thing you know, Fuchs is missing and MacReady leads the hunt to find him.