Jose Aldo is considered one of the pound-for-pound best fighters in the world. However, he is not that well-known due to the fact that he fights at 145 pounds. Up until last week, he was in the WEC, but due to the merger between the UFC and WEC, he is now the UFC featherweight champion.
Choosing Aldo's first fight as the UFC featherweight champion to be the co-main event at UFC 125 makes sense for several reasons.
First, the UFC's New Year's card is typically one of the better cards of the year and therefore draws in many viewers. This is a great benefit to Jose Aldo who is an unknown to many casual fans because they will see him when they tune in to watch Frankie Edgar try and defend his UFC lightweight title against Gray Maynard.
Also, there are notable fighters such as Clay Guida, Nate Diaz and Chris Leben on the card. If the UFC does a significant number of buys for UFC 125 and Aldo wins an entertaining fight, he could easily be headlining the next PPV he is on.
Secondly, Aldo-Grispi is not a bad choice for a replacement main event if Edgar suffers an injury and is forced off the card. Depending on when the injury takes place, Aldo may have also already picked up a popularity boost from the UFC hype machine, which would make even more sense for what should be an entertaining championship fight to be the main event.
Another reason Aldo-Grispi makes sense as the co-main event is that Aldo has already expressed his desire to move up to lightweight and challenge for that title as well after cleaning out the featherweight division.
Another interesting aspect of changing weight classes is if Edgar loses he could easily announce the drop to his more natural weight class of 145 pounds and call out the loser of the co-main event or another of the top contenders at 145 and bring added credibility to the division.