I would say honestly, it will take more than a couple weeks of rehearsals for Simon to fit in. I'd say, by a few weeks in, and maybe even next leg, he will be killing it. It just takes time. You have to remember that Nicko has been playing some of these songs for 42 years. That's a LOT of muscle memory. And of course a bit more used to playing these massive shows. From what I gather, Simon is very experienced, but jusyt maybe not in massive settings like a Maiden gig. He will get there, 100%.
The thing with this is, Nicko is by for one of the most identifiable drummers, ever. You hear him play and it's like 'Yep, Nicko' straight away. It's weird because unlike a guitar, where you have vibrato, picking style etc., drums technically sound more neutral. Many famous drummers, I can't tell the difference just for listening. But you can with Nicko. My thoughts are that it is going to be very difficult to get used to not hearing Nicko's style. But that is a different subject than whether or not Simon is capable of keeping Maiden driving forward. I think hearing Simon on these first two gigs has been genuinely awesome. He's nailing 99% of the parts. Other parts (Like the Murders intro) will fall into place.
I really like Simons playing - no detraction from Nicko. Just a bit of a different style that might take a bit to get used to. Honestly, some of Simon's tom fills are sounding excellent - Powerslave for example.