If you are not using a person's picture or a licensed image from a team or league, there is no issue I can see. A player's picture can not be used on a card if they have not signed an agreement (usually through the league's players union) to have their image used, and the league would have to approve (and you'd have to pay for) the use of any team and/or league logos/images.
If you use a card such as the one above and just have a random player sign it, I can't see the issue from a legal standpoint. Unless the technology used to create the card is copyrighted by Topps/Panini/etc., then you may have an issue. So if you produced a Superfractor Card and Topps has a patent on that specific image, you could have an issue.
But a standard sports card sized piece with a football and a line for an autograph is not an issue from a legal or copyright infringement viewpoint that I can see. But would a player look at it funny if presented with it?
Collecting Autographs on 2014 Topps Pro Debut: 163/220=74%
Collecting Bradley Roby... and I don't know why!
Collecting Steve Garvey