Punk's 2nd fight, if he winds up having one, could depend upon a few factors.
1. The quality of his first opponent and the quality of his performance against said opponent - IF they go with the formula they used with Brock Lesnar, Punk's first fight could be against someone with a pretty solid record but isn't a mega star. In Lesnar's UFC debut, his opponent was Frank Mir and Mir had a record of 10-3. Mir had also been former UFC Heavyweight Champion but lost the title in his first defense, which happened to be his first fight in fairly close to 2 years. If Punk fights as a middleweight, I could see his first fight being against someone with a respectable career in UFC but who also, probably, has the best days of their career behind them like Michael Bisping, Rich Franklin, Lyoto Machida and a few others. If Punk fought any of them and won, it'd be a pretty meaningful win as they've all had a good deal of success in UFC.
2. How much money the ppv draws - Brock Lesnar's debut bout took place at UFC 81 and was advertised as a double main event, the other main event being Tim Sylvia vs. Antonio Rodrigo Nogueira, and it drew 600,000 ppv buys. If Punk's debut does draws that or more and he has a strong showing, it's possible they could hotshot him straight into a title shot very quickly. After Lesnar's first fight in UFC, which he lost, he was put up against Heath Herring, who was pretty much a nobody, he won that fight and found himself facing Randy Couture for the UFC Heavyweight Championship 3 months later.
I know you said if but I highly doubt they go the same route they did with Brock Lesnar. That dude is a freak of an athlete that basically could've done anything contact sports related due to his size and athleticism. A true once in a lifetime type specimen.
I mean we're talking about a guy that made an NFL practice squad after leaving WWE, and he was also a former NCAA wrestling champ, and at the time had physical advantages over everyone in UFC. I mean he beat a up much smaller aging Couture for the Championship and the heavyweight division was considered very weak in comparison to other divisions at the time.
So a bit of a different situation for Punk, he hasn't won any bjj tournaments and isn't a freak of an athlete, however he is a good mic worker so I expect some good shoots and should be entertaining if it happens.