Simone Magill, who plays for Northern Ireland, injured her knee in Thursday's opener.
Magill was helped off the pitch in tears in the 78th minute of her side's defeat.
He would be a big loss for the club.
"We're concerned about that more than anything," said Shiels.
She has a knee. I'm not a doctor but I don't think the signs are good.
The game was Northern Ireland's first at a major tournament.
Andy Gray is a sports reporter for the British Broadcasting Corporation.
You were worried by Magill's reaction to coming off that it was a bad injury, and Shiels confirmed the news in his post- match press conference.
It is a huge blow to Northern Ireland. The focal point of the team is Magill and he gives them a key attacking outlet and WSL experience.
The two out-and-out strikers in Northern Ireland are just 20 and 19 years old.
It's a cruel blow to a player who was set for a fresh start at Villa and everyone hopes she can make a quick recovery.
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