I fail to see the problem..
There can be no duplicate hero's in the League, ergo, if you have less than 10 owned, you risk not being able to chose a hero.
Not to mention that it's so easy to get to 10 hero's owned. I have 7 hero's at level 20. I will easily get to 10 by level 30 (Even if I don't think I'll be playing Hero League)