The Spanish full-back moved to the Stadium of Light in January on a short-term loan from Italian side Fiorentina.
He did enough over the second half of the season, as Sunderland avoided relegation out of the Premier League, to convince Poyet that he would be a welcome permanent addition.
Talks have been opened regarding a possible summer switch, but no deal has been done as yet – with reports in Italy suggesting that Fiorentina are now looking to keep the 23-year-old.
Poyet said in the Sunderland Echo: "It's close, but not close enough. Everything is on track.
"It's just like I explained before. These deals for everything to come together, so many people need to be happy. It's not that easy.
"We need to look at other things as well because depending on how much you spend on a player, it depends what else you’ve got.
"We're trying hard. It's been a bit more difficult than I expected, but we're not going to stop."
Poyet admits he is eager to get deals over the line as quickly as possible, with 12 players having left Sunderland this summer and only three added to the ranks.
"It's difficult to say how much progress has been made in a number – 50 per cent or 75 per cent – but we're trying," said the Uruguayan.
"Days go past and there are things that we need to do quickly because Portugal (training camp) is approaching and we want most of the squad in there.
"My idea was to have probably 20 of my 23-man squad in Portugal. That's not the case.
"I'm working for it and I'm believing, until that changes. It's just an ongoing situation which is not easy.
"The market is only on with big, big signings and big transfers. The rest are all waiting, I don’t know why.
"It's not my job, so I think it's better if you ask (sporting director) Lee Congerton."