This reddit post was informative:
Yours-Sincerely 45 points an hour ago*
Mexican here. The field has been a complete disaster for months now (July to be precise).
What happened?
The natural grass of the Azteca stadium was replaced with a hybrid grass mix in July. This hybrid grass needs a minimum of 15 days for the roots to settle in, however for some unknown reason, the grass was installed ONE DAY before the first match of the Mexican soccer season. So, as a result, the grass was never able to settle and it's been a disaster ever since.
What's worse is that since two Mexican teams use the Azteca stadium as their home, that means that there is at least one soccer match played each week. Then you add concerts and other events, and the damage is multiplied.
What has the impact been?
Soccer players and coaches have been complaining about the dangerous conditions since July. Although to the best of my knowledge, there haven't been any injuries caused by the field, the quality of the matches is usually poor as teams are reluctant to risk getting injured.
What has been done to fix it?
Tarkett, a French firm specializing in stadium maintenance (including with Liverpool, Arsenal, Chelsea, amongst others) was brought in during the August-September time frame to try to fix the issue. It seemed to improve after a 2-week period without games but went back to its terrible condition soon afterward.
What do I expect to happen with MNF?
From what I know, it doesn't seem likely that the NFL pulls out given that we are just a few days away. It makes me sad that this incredible match-up will be impacted, and the Azteca (and Mexico) will reflect poorly. Make no mistake, the NFL is HUGE in Mexico (to many people's surprise), and the Azteca is one of the most legendary stadiums in the world. I hope that we see a fantastic match-up, no injuries, and that we redeem our reputation next year.