The 22-year-old was forced to withdraw from the Wales squad for Wednesday's friendly against Austria at the Liberty Stadium.
Ramsey has returned to Arsenal for treatment and could miss this weekend's trip to the Stadium of Light.
A Football Association of Wales statement on Wednesday read: "Aaron Ramsey has this afternoon withdrawn from the Wales squad to face Austria at the Liberty Stadium in Swansea.
"Aaron has a problem with his calf and has returned to his club Arsenal for treatment."
Midfielder Ramsey has made 32 appearances for the Gunners this season, scoring once.
Wales boss Chris Coleman does not believe the injury is too serious, although he is unsure if Ramsey will be fit for the clash with Sunderland.
"I don't know if he will be ready for the weekend, it's a tough one but maybe if his calf settles down," said Coleman, speaking after Wales' 2-1 friendly win over Austria on Wednesday night.
"We maybe could have played him tonight but if something happens and we send him back injured, it is not ideal. We are trying to build relationships with clubs.
"When the doctor looked at it and red-flagged it, we could not take that risk. He may have a chance for the weekend, but I don't know."
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