Dick Advocaat’s growing discontent with life at Sunderland is reaching the stage where it would be no surprise if he decided to resign.

Much depends on whether Ellis Short, the club’s owner, strengthens Advocaat’s squad before next week’s transfer deadline. “I am not happy at all,” said Sunderland’s manager as he approached Tuesday night’s League Cup tie at home to Exeter City. “We have to improve and we can only improve if we get some other players in. We really need two or three more players. I have told everyone and I will keep saying it until the last day – but not much has happened.”

Advocaat exonerated Lee Congerton, Sunderland’s sporting director, from blame but, perhaps significantly, did not mention Short or Margaret Byrne, the chief executive. “Lee Congerton is doing a great job with little money,” he said. “We really appreciate what he has done so far – but we need more.”

It is clear that the much-travelled former Holland manager is not exactly delighted with his current personnel. After abject surrenders at Leicester and at home to Norwich in the season’s opening games, Sunderland improved markedly in securing a 1-1 draw with Swansea at the Stadium of Light on Saturday but Advocaat remains unconvinced as to the team’s potential.

“The last line [the defence] has to improve,” he said. “We put them together this morning [in training] and the co-operation on some set pieces was very poor. Instead of watching the ball and the man, sometimes they were sleeping and you don’t know why. We can only do better but the first two games were very bad.

“The difference with a team like Swansea is that at Swansea everyone knows if they have to go this way or that way. The way they operate is natural. But sometimes we have to tell our players what to do. In my opinion, if you play at this level, you should know what to do.”

When Advocaat was asked if the current squad was capable of avoiding relegation and enjoying a cup run there was a long pause. “Do I have to give an answer to that?” he said. “I think it will be very difficult. Last year we were lucky to stay up. Almost every other team is improving if you see the investments they have made, especially to the front line – there is mammoth inprovement.”

He appears concerned by Short’s enthusiasm for shopping in the bargain basement area. Jeremain Lens, an £8.5m signing from Dynamo Kyiv, shone against Swansea, as did Yann M’Vila, who is on loan from Rubin Kazan, but the summer’s other signings, Adam Matthews (£2m from Celtic), Younès Kaboul (£3m from Tottenham) and Sebastián Coates (£2m from Liverpool) are yet to impress. Meanwhile Connor Wickham has been sold to Crystal Palace for £8m.

“In the last 20 minutes against Swansea Lens showed what you get if you buy a player for that price,” Advocaat said. “All the other players are bargains. Some people still believe in numbers, instead of quality. But a player of that calibre £8m-£10m, you know what they bring.”

Asked if Sunderland were finding the transfer market a difficult place this summer, he replied somewhat cryptically. “It’s as hard as they want,” he said.

Another comment he made could be interpreted as a plea for Short to place his trust in both the manager and Congerton: “If you bring in the right players and the right managers and the right people – and I think they are here now – and you trust them and you have some resources to do something, knowing, and I wasn’t intending to say this but I will say this, because I don’t care, knowing what will come in the next three years [the new TV deal], I would do it differently, let’s say that,” he said.

Jonathan de Guzmán’s agent has said his client could leave Napoli to join Sunderland on loan. The 27-year-old midfielder made 12 Serie A starts for Napoli last season. And Fabian Parisi claimed a move to Sunderland was possible, though Advocaat has shown no clear interest.