Chris Coleman has revealed he hopes to have made his first signing by Sunday.
It is understood the player is a young central defender from Premier League side Chelsea, with Coleman keen to bring in up to five fresh faces this window.
Coleman refused to confirm any names but it is understood the player Sunderland are closing in on is promising Chelsea academy player Jake Clarke-Salter, 20.
The Black Cats boss, speaking after his side's 2-0 FA Cup third round defeat to Middlesbrough at the Riverside, said he was confident the deal would be done.
It was expected to go through on Friday but was delayed after the player's parent club decided to use him in the FA Cup today.
The Sunderland boss wouldn't confirm the name of the new signing but did rule out signing Chelsea and Wales defensive midfielder Ethan Ampadu.
Coleman said: "I am hoping he doesn't get injured and we should be okay for what should have been done yesterday.
"There was a change of plan but I am hoping somebody will arrive tomorrow.
"It is not Ethan Ampadu. I have heard that but it isn't him."
When asked if there were any other deals close, Coleman added: "Possibly. Let's wait and see but I am pretty optimistic."
The Sunderland boss was also asked about former striker Lewis Grabban, who has returned to parent club AFC Bournemouth after his loan deal was cancelled by mutual consent.
The striker, Sunderland's top scorer with 12 goals, had made it clear he wanted to return to the south coast, with interest in him from Fulham and Wolves this month.
The Sunderland boss says he won't go down on his hands and knees to beg any player to stay at the club and says he only wants fully committed players.
Coleman said: "He scored a lot of goals for us but we didn't get a lot of wins, if you look at the stats.
"Consolations and penalties yes, he is a natural goalscorer but we won one game in 16. Of course you don't want to lose that but that is up to him.
"If he doesn't want to be with us, then fine. As I said from the start, why should I go down on my hands and knees to convince anybody to be at Sunderland.
"Even though we are bottom, yes fair enough it is tough. If he doesn't want to be here then he goes back to Bournemouth and we get on with it.
"It is an area we are looking to sort out [up front]. I think maybe possibly more than one in that area, we might have to look at.
"We will wait and see. I said if anybody wants to be in this fight with us then you have to make sure you are all in.
"You can't just dip you toe in the bath water when you feel like it."