I was worried he might lose the keys he put in his pockets.
I was a bit concerned his keys might fall out of his pockets.
この場合、次のような言い方ができます。
ーI was worried he might lose the keys he put in his pockets.
「彼がポケットに入れた鍵を失くすのではないかと心配だった」
ーI was a bit concerned his keys might fall out of his pockets.
「鍵が彼のポケットから落ちるのではないかとちょっと心配だった」
「…と心配だった」は I was worried や I was a bit concerned を使って言えます。
ご参考まで！
カナダ 
