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The 24-year-old centre-back was stretchered off during the Championship clash and is currently undergoing assessment to determine the full extent of the injury.
QPR head coach Julien Stephan admitted the early signs are concerning, with the injury dampening what was otherwise an impressive afternoon for the west London side.
Speaking to BBC Radio London after the match, Stephan said: "We don't know exactly what the situation is with his knee, but it's never good to see a player like this come off with a lot of pain. It's never a good sign."
The Frenchman revealed the incident had overshadowed QPR's emphatic win, despite his satisfaction with the team's performance.
"Normally, when you play a game like this and you score four, you are very happy and very positive – and I have to be very positive about the way we played," he said.
"Unfortunately we had the Amadou situation and that's why it's a mix of feelings for us."
Mbengue remains a familiar figure to Reading supporters after spending three seasons at the Select Car Leasing Stadium. The Senegalese defender made more than 100 appearances for the Royals following his arrival in 2022 and established himself as a popular and dependable presence in the back line.
After departing Reading at the end of his contract in the summer of 2025, Mbengue joined QPR and has become a regular feature in their defence.
QPR are expected to provide a further update once the results of the defender's scans and assessments are known.