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Manager Paul Jewell admitted that first impressions suggested a "serious injury" although the hospital tests would reveal the full extent.
An absence of any length for the former Denmark international would leave Latics short of bodies in the engine room.
Jason Jarrett's still on the road to recovery from a broken leg and three other midfielders – Mike Flynn, Paul Mitchell and Tony Dinning – are out on loan.
But the signing of Gareth Whalley last week looks a timely one and he could get his first team chance sooner than expected.
Jewell said: "What happened to Per was a bit of a downer and we'll have to see what the X-ray and scan shows.
"It's a problem with his right knee and it's a major worry as it looked like a serious injury."
Latics had the game won by the time Frandsen left the action, three goals in six minutes during the first half seeing to that.
Nathan Ellington scored two of them either side of a Jason Roberts strike, and Roberts went on to add a fourth goal in stoppage-time.
Jewell was delighted with the win which sent his side back to the top of the Championship.
He said: "The first 20 minutes were pretty fraught without a lot of quality.
"Once we scored the first goal you could see the confidence coming back – if it had ever been away.
"We then looked very dangerous every time we went forward and were in full control.
"In the second half, we knew United would come at us, but I can't remember them having a chance until right at the end from a corner.
"To win 4-0 against a good side, not even I can have too many complaints."
Turning to two-goal Ellington, Jewell added: "Without beating our own drum, I think Duke is twice the player than when he first came here.
"He is fitter, stronger, mentally stronger, and is a better footballer.
"The lad will always be unpredictable but now he is getting better as a team player. He's outstanding in the air, has two good feet, he's quick and has got all the right ingredients.
"Duke was one of those players who everyone looked at when he was at Bristol Rovers, and he was always touted in the lower divisions.
"I'd always liked him, he's raw – he still is – but there's a lot more to his game than there used to be.
"He and Jason Roberts are best of friends off the pitch, and I think they are Siamese twins at times!
"But you have to be careful not to take the shine away from the team because the two strikers will be the first to tell you that it is a team game.
"They get all the headlines and are doing exceptionally well, but they have some quality players around them too."
Jewell changed his starting side for then first time in the league this season, dropping David Wright and restoring Nicky Eaden to right-back.
He said: "Wrighty may feel unfortunate but I felt Nicky's use of the ball gave him the edge on Saturday.
"Wrighty's last two performances have been lacking a bit of confidence.
"But when he came on into the midfield role he did very well and has obviously has more strings to his bow."