Swansea City midfielders Kris O'Leary and Owain Tudur Jones could be out for a month with ankle ligament damage.
Tudur Jones has made a great start to his Swansea City career
O'Leary was injured in training while Tudur Jones was carried off after an hour of Wales' Under-21 match in Poland on Tuesday.
The 20-year-old went for an X-ray after the 3-2 defeat, but it showed no break.
It is an untimely blow for Tudur Jones who has forced himself into the Swansea line-up after his summer move from Welsh Premier side Bangor City.
He had started Swansea's last four matches and won high praise from Wales Under-21 manager Brian Flynn after his performance against England last week.
Said Flynn: "I was delighted with Owain's full debut for the under-21s against England on Friday.
"And when you look where he was three months ago, still at Bangor, it shows what can be achieved if you want it.
"He has that desire. My first impressions of him is that he so desperately wants to be a professional footballer, and he will do everything to grasp the opportunity now it has come along for him.
"I hope he does and makes it to the top. Players like Mark Delaney have done it after being in the Welsh league."
Swansea manager Kenny Jackett is hoping to fill the midfield void left by O'Leary and Tudur Jones by signing Marcus Bean on loan.