Cashin’s injury comes at ahead of a busy festive period, with the Rams playing four matches in 12 days up until 1 January.

The fixtures start with Friday’s trip to Luton Town, and includes home games against West Brom and Leeds United before they travel to Sheffield Wednesday.

Cashin has been instrumental for Derby in recent seasons, first establishing himself in the backline under former boss Wayne Rooney in the second half of the 2021-22 season when they dropped out of the Championship while in administration.

He went on to win the club’s young player of the season award in their first campaign in League One, and started 43 of their 46 matches as they won promotion back to the second tier last season.

The defender has featured 20 times for the Rams this season, and his goal against Pompey last week was his first for 10 months.

“It’s a blow and it’s a blow for the kid as well because he was in a really good vein of form,” Warne added.

“He’s been a big part of the success we’ve had. It’s disappointing but hopefully the lads perform admirably without him on Friday.

“No-one wants to lose their players, their consistent performers, but then I can’t cry about it because in fairness to him and Curtis Nelson they have virtually played every minute for a season and half.”

Meanwhile, Warne has said Derby will expect to have defender Ryan Nyambe sooner than expected.

The Namibia international was ruled out for up to six months with a knee injury in October, but could return in late January.