「申し訳なさそうな表情」は
ーHe looks like he feels bad.
ーHe looks like he's sorry.
を使って表現できます。
例:
My teacher said, "I'd love to continue talking with you, but we are running out of time" with a sorry look on his face.
「先生は、話を続けたいけど、残り時間がわずかです、と申し訳なさそうな表情で言いました」
with a sorry look on his face を使って「申し訳なさそうな表情で」
ご参考まで!