Midfielder Chris Herd has returned to Aston Villa from his loan spell at Wigan Athletic to have surgery on a medial ligament injury.
The 25-year-old was taken off on a stretcher in the Latics' 1-0 win at Reading on Tuesday.
The Australian joined Wigan on loan in January until the end of the season and had played three times.
"He's been a terrific servant since coming into the club and desperate to play football," said boss Malky Mackay.
"Unfortunately he fell on his knee and his medial ligament has snapped and he's going to need surgery.
"He is going to have to go back to Aston Villa and leave us."
With recent injuries to other players in the squad, Mackay, 43, has hinted they will look to bring in another play to cover for him.
"Off the back of the Grant Holt and Emyr Huws injuries and Juan Carlos Garcia's situation, it's an unforeseen blow but one we've got to get up and get on with," he told BBC Sport.
"We have to try and look at that area to strengthen as it does bring us down to bare bones."
Mackay also confirmed former Arsenal, Liverpool and Stoke winger Jermaine Pennant is training with them.
The 32-year-old was released by the Potters last January and had been playing at Indian Super League side FC Pune City.
"He's been in for a couple of days so we'll see how that goes," said Mackay.
"He has come in with a nice fresh approach, he's desperate to do well and show people he's still a top quality footballer.
"He's been out in India playing and there is a possibility of him going to Thailand but that didn't transpire so he's asked to come in and train."