User Selected Start of Season
While most leagues are created and drafted prior to the start of the NFL season, NFL.com gives you the opportunity to either draft or create your league after this date and still have statistics begin with the kickoff of the NFL regular season.
When creating your league, you have the option of starting your season with Week 1, 2, 3, 4 or 5 of the NFL regular season. By default, your league will begin and matchups will start with the next week after your league is drafted. For the vast majority of leagues, this will be Week 1, including if your league is created, filled and drafted at any time prior to kickoff of the NFL regular season.
Note that when retroactively starting your season in a previous week, teams' starting lineups will be based on the default lineups that would have been filled out post-draft. If you picked up a player after your draft (and after the retroactive league start date), these transactions won't reflect on the prior week matchups, but the actual matchups will still take place based on your players' stats in the NFL games. Now you can play a full season of fantasy football even if your league comes together late.
Likewise, if you do decide to start your season at a later date, any transactions will be valid, but will not reflect on a matchup's score until your actual fantasy league season begins. Any adds made in weeks before the start of your fantasy season would still count, but they simply would not result in changes to a starting lineup in those weeks so no matchups occurred.