I prefer to address the opposite question: Which sports is easiest? The OP asked about professional so we have to include all the minor leagues and all the sports. In the US, there are many minor leagues and teams: AAA, AA, A, instructional leagues, legion ball, etc. and the rosters are larger than basketball. It just seems to me there are more opportunities than most other sports (maybe there are similar opportunities in soccer overseas? I don't know). Now the trick is, how can you be a pro baseball player, but not have to consistently hit a fastball well? I've always felt the answer is: Be a catcher. It's a skill you have to learn, but they always seem to be in demand, and you're not expected to hit for high average.