You can drop either the "When he entered MLB" or "in baseball".

"When he entered MLB, many experts doubted whether..."
or
"Many experts in baseball doubted whether..."

Then the second half of the sentence you say "genius" referring to a baseball player. This isn't chess, it's a physical challenge with specific tactics and skills. I wouldn't say "genius" applies unless he was revolutionizing those tacitcs (cutoff men, backups, etc). You could say that they "doubted whether he could succeed in the league" or something of that nature. Also drop the ? mark.

For the second sentence, go with the first part. Then use his accomplishments to show why the doubters were wrong. The second part of the second sentence just doesn't flow.