Stoke City have terminated the contract of Uruguayan midfielder Diego Arismendi by mutual consent, the club have announced.
The 24-year-old is returning to South America after making just five first-team apperances since signing for the Potters from Club Nacional in August 2009.
Arismendi spent the whole of last season on loan at Barnsley and also had spells with Huddersfield and Brighton.
Stoke's Chief Executive Tony Scholes said: "It has been an incredibly frustrating time for everyone because he came over here with such high hopes of making a name for himself in English football, but it clearly hasn't worked out that way for him.
"His contract with the Club would have come to an end next summer, but with him not in our 25-man Premier League squad and little prospect of another loan move, it was in the interests of both parties for us to terminate his agreement now.
"Diego will be returning to Uruguay immediately, so we would like to wish him all the best in finding a new club to get his career up and running again."
The Ulsterman will mark 12 months at the helm on Thursday, by which point he will hope to have addressed a difficult start to the season which has left some questioning his presence on Wearside.
O'Neill continues to enjoy the support of owner and chairman Ellis Short despite last weekend's unfounded rumours that he had tendered his resignation, and as he prepares for Sunday's Premier League visit to former club Norwich, his determination to build a brighter future for the club he supported as a boy is undimmed.
He said: "It's almost exactly what I thought it would be and from that aspect, it hasn't disappointed.
"It's a tough job, I know. Mr Short said to me when I came here in first of all, 'Look, try to stay in the division', and perhaps we got carried away with ourselves because we were able to achieve that more quickly than I had thought.
"This season, it has been tough - I expected it to be tough, and obviously, it's important that we are in this division for next season as well.
"There is no such thing as a five or 10-year project anymore in management, but it takes a little bit of time. That's very, very obvious, but I expect it to be successful."
O'Neill got off to a flying start in his latest job when he dragged the side he had inherited from predecessor Steve Bruce out of a relegation fight in impressive style and then set about the task of ensuring they avoided being sucked into that kind of dogfight again.
But almost a year on and having invested £22million in Steven Fletcher and Adam Johnson during the summer, the Black Cats are hovering precariously over the drop zone having won just two of their last 21 league games.
Major investment in January was never really on the agenda, but it may well be now, and O'Neill admits he would not be averse to carrying out significant strengthening work during the notoriously difficult transfer window.
He said: "It is the one opportunity during the course of the season that you get, and it would be inappropriate for us not to have a look at that.
"We will try to strengthen if we can do. Most people will tell you that maybe it is not the best time, but I have done it in the past and the players I have taken in in the January transfer window have proved pretty successful, the likes of Ashley Young (at Aston Villa).
"If you were thinking about somebody that you feel is capable of doing something pretty okay for the club for the next couple of years, then January is no different to the summer time."
In the short term, O'Neill will head for Carrow Road, where he had two spells as a player and one, briefly, as manager, without central defender John O'Shea and facing up to the possibility of losing skipper Lee Cattermole for months rather than weeks.
The midfielder was visiting a specialist in London today to have suspected ligament damage assessed.
O'Neill said: "Lee Cattermole, as we speak, is actually down with the specialist in London at the moment and we haven't got the proper update yet from them, but he is going to be out for some time."
In the circumstances, Sunderland may consider reviewing Republic of Ireland international David Meyler's loan spell at Championship Hull, which is scheduled to run until January 1, with the 28-day period during which there is no right of recall due to expire.
O'Neill said: "The 28-day period will soon be up and it's something that we would probably have to have a look at. We look at all sorts of options that we have."
