Hien sat down in his own penalty area after a duel and took one knee. The centre-back was then led off the field and substituted.
"It looks bad," national team captain Graham Potter told SVT.
Sweden's goal scorer Anthony Elanga had not had time to talk to Hien when he met the media in the mixed zone.
"I haven't seen him yet. After this I'll go and see him. He's an important player for us. But those who came in helped a lot, and you know what that means: we're 26 players, so everyone has to be ready when they step on the pitch," Elanga said.
Goalkeeper Jacob Widell Zetterström:
"I saw him at halftime, actually, and again after the game. Very sad, of course, something you never want to see. In our group we support each other in good and bad times. Even though it probably feels really tough for Isak, he knows we are here."
The point against Japan was enough to take Sweden through to the round of 16.
