McDavid did 3 crucial things, within the span of literally three seconds, that completely handcuffed Rielly on this play:
1) He came into the zone with some speed, and then slowed up almost stopping. This is why Rielly, who literally just jumped onto the ice BTW, is caught flat footed. He can't skate toward McDavid at this point because that's a recipe for disaster, and he can't start skating backwards because that just gives McDavid more room to move. The clock at this point says 8:38 and I don't really see how Rielly could have played this better.
2) He sells the pass to a teammate at the other end of the boards. I don't even really think this messed Rielly up as much as others do. He doesn't overreact to the pass, he doesn't give McDavid anymore space to the inside of the ice, he keeps his eyes on McDavid the entire time. But mentally he has to give some respect to a potential pass. Even still, he's playing McDavid body on body and stick on stick and protecting the inside of the ice. The clock at this point says 8:37 and I don't really see how Rielly could have played this better.
3) Remember when I said McDavid slowed down? Well he makes his second speed change in as many seconds and goes from 0 to 60 this time as well as probably anyone in the history of the NHL could. Rielly at this point is screwed. He has his stick on the wrong side of McDavid, he's flat footed, and he's skating backwards against the best skater/player in the league who just turned on his jets. You can say he looks like an idiot there but McDavid forced Rielly into all 3 of those positions because a second ago that was the best way to defend the situation in front of him. Then McDavid just went in for the kill. This happens at 8:36. Again, he hit the blueline at 8:38.
There's no other hockey player with the combination of smarts, skill, and speed who could have pulled all that off in 3 seconds.