First of all, it is perfectly possible to watch Monday Night Football on Roku because it is compatible with Sling TV, fuboTV, Hulu With Live TV, DirecTV Stream, and YouTube TV, you just need to subscribe to them and then in the appropriate app on Roku, you will need to log in to your subscription account and follow the instructions to browse and select the soccer game you want to watch.
Finally, watching Monday Night Football for free on Roku depends on whether Sling TV, fuboTV, etc. offer a free trial.