Sheffield United manager Chris Wilder says defender Jack O'Connell could be out for the season with a serious knee injury.
Jack Robinson took the place of the 26-year-old in Sunday's Yorkshire derby against Leeds.
"Jack O'Connell is possibly out for the season and probably needs a knee operation," Wilder told BT Sport.
"He tried to come back and then had to have a scan. We had to adjust pretty quickly and he established himself in the Premier League last season. It is a huge blow for him and us."
O'Connell played 33 times in the Premier League last season and has made 177 appearances for Sheffield United in all competitions since joining the club from Brentford in 2016.
Summer transfer window - key dates and times
The summer transfer window closes at 11pm on Monday October 5 with a feast of activity lined up on Transfer Deadline Day on Sky Sports News and across our Sky Sports platforms.
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