This fight is all about distance. In short, McGregor's critical distance is slightly farther out than Aldo's. McGregor wants everything to start from long range. He wants space to throw his hipster kicks, and he wants to be able to step into his left hand when you're on the defensive. He also wants enough space that if you step forward, he has plenty of time to take an angle and counter you. Ideally, he wants his opponents moving away from him when he attacks, and forward when he's setting a trap. Aldo is famous for his low kicks, but his hands are what have been winning many of his recent fights. He's one of the best boxers in MMA. He's dangerous with both hands, and is ideally throwing his punches from just barely within the pocket. Occasionally he'll step a bit deeper into the pocket to knee, or step all the way inside to take the other guy down, but his real critical distance is just at the edge of and inside the pocket.
It's important to note that both guys are very skilled at distance and in the pocket, so there's some overlap between each of their usual critical distances. However, Aldo is most likely to win the fight if he can force exchanges to take place just a step closer than McGregor wants them to. His best chance of doing that is to step forward when McGregor does. If Aldo backs up every time McGregor kicks, he's gonna get stuck too far away to work his hands. He should be able to counter some of McGregor's kicks at distance with his own, but ideally he'll step inside them to counter. If he can do that, he'll have to deal with McGregor's left hand. Again, he doesn't want to move away from it. Guys who run from McGregor get run over. He needs to step forward, move his head and make McGregor pay when he throws that left. If he can do that, and he can start stepping into the pocket to set up his own offense (preferably throwing outside low kicks and inside body kicks when McGregor backs off), then he should be able to force McGregor to respect him, slow down the pace of the fight to one that suits him, and take control.
However, McGregor wants to do everything he can to keep Aldo at range, and make sure that any time Aldo steps forward, it's into a counter. Not only that left straight, but I'm fully expecting McGregor to try a couple classic tkd jump spin kicks to the body as counters when Aldo comes forward looking to throw hands. The more McGregor can force Aldo to initiate his offense from too far away, the more he can wear Aldo down. The more he can back Aldo up, the more he can unload.
So in this fight, control of critical distance is key to control of pace, which will ultimately be the deciding factor IMO.