Sunderland manager Simon Grayson says his side must “knuckle down” and put in the hard yards ahead of their Championship opener at home to Derby County on Friday.

The club’s pre-season came to a shuddering end last Saturday when they were thrashed 5-0 by Scottish champions Celtic at the Stadium of Light in the Dafabet Cup.

To make matters worse for the Black Cats, midfielder Darron Gibson was filmed by a supporter following the defeat criticising certain players while also making comments about former boss David Moyes.

Group harmony appears to be at an all-time low for the north east outfit and the players must respond in the tricky home clash against the Rams.

Grayson’s men, who are 17.00 to win the Championship, are still narrow 2.38 favourites, the draw is priced at 3.25 while the visitors, who are 13.00 to lift the league title, are a 3.10 chance.

Sunderland’s new boss, who left Preston this summer to take the job, admits there is still a huge amount of work to be done and the players need to bounce back from the Celtic loss.

He told safc.com: “We will knuckle down and make sure the thing we did hugely wrong against Celtic won’t be there on Friday.

“It is a reminder to everyone that we are nowhere near finished. This is not going to happen overnight with the project that we are on.

We will dust everybody down and we will start our preparation ready for the real start of the season.”