The midfielder left the field on the half-hour mark of this English second-tier match and could be ruled of Wales' Nations League matches against Belgium and Poland.
It was an injury to the hamstring in Allen's other leg that hampered his start to the current campaign following a pre-season move from Stoke to the Swans.
"It's his hamstring, but it's not the one he has done before. That's why there's not a huge amount of concern at the minute," said Swansea manager Russell Martin.
"Joe doesn't feel it's a bad one. I think he has a good understanding of his own body and he is hopeful he came off as more of a precaution.
"He felt something, rather than it being a serious injury. We won't know until we get him scanned, but I would have thought it gives him only a slim chance of being involved on international duty this week."
Martin added: "The main focus with the international players we have is to try to make sure they are on the plane in November. Joe is really important for us and for Wales, so for everyone's sake let's hope it's nothing too serious."
Wales have qualified for their first World Cup finals in 58 years. They play the United States in their tournament opener in Qatar on November 21st, ahead of further group games against Iran and England.
