If it had been a young player, he would probably have been sulking. But Kazu was able to seize a rare opportunity that came along his way!
ご質問ありがとうございます。
"If it had been a young player,"=「もし若い選手だったら」
"he would probably have been sulking."=「ふてくされていただろう。」
"But Kazu was able to seize a rare opportunity that came along his way!"=「でもカズはまれなチャンスを掴むことが出来た!」
ご参考までに。