O'Neil's future was plunged into doubt on Monday when financial motives led to his omission from the Boro squad that secured a 2-0 win over Plymouth, and the former Portsmouth midfielder now appears certain to be on his way out of the Riverside.
He will not be alone, as an anticipated failure to bounce back to the top-flight at the first time of asking will result in a further cull of the squad Gordon Strachan inherited last October.
Boro are expected to listen to offers for defender David Wheater in order to raise funds for Strachan's rebuilding project, while loanee Stephen McManus is poised to return to Celtic following his permanent employers' dismissal of former manager Tony Mowbray.
Jeremie Aliadiere, Emanuel Pogatetz and Chris Riggott are all due to become free agents at the start of June, and it is unlikely that any of the trio will still be Boro players at the start of next season.
The situation will change if the Teessiders force their way back into the Premier League via the play-offs, but that looks unlikely with a six-point gap separating them from sixth-placed Leicester City.
While the financial effects of a second season in the Championship would be tempered by a final parachute payment from the Premier League, further cuts would be needed if Strachan was to embark on a spending spree of his own, something the Boro boss clearly desires given his recent comments about the inadequacies of the current squad.
O'Neil and Wheater are likely to be the two players jettisoned first, as they would command a larger transfer fee than any of their team-mates.
While Strachan claimed O'Neil's situation would be assessed on a match-by-match basis on Monday, he appears to have played his final game for Middlesbrough.
Both Blackburn and Fulham have previously expressed an interest in signing the 26-year-old, and it is hard to imagine Boro officials refusing to pay the final £1m instalment of O'Neil's transfer fee if they did not intend to move him on in the summer.
Similarly, it is hard to imagine them refusing to listen to offers for Wheater, who remains a coveted figure despite a somewhat patchy season.
The Redcar-born defender has previously been linked with a move to Everton and could conceivably command a transfer fee of around £4m.
Strachan's stance over Wheater could yet be influenced by what happens with McManus, as the Boro boss is keen to turn the Scotland international's temporary switch from Celtic into a permanent transfer.
Had Mowbray stayed at Parkhead, such a move would undoubtedly have been an option.
But with Mowbray's temporary replacement Neil Lennon desperate to restore McManus to his own club's ranks if he is appointed on a permanent basis in the summer, it now appears all but certain that his time on Teesside will not be extended.
I'm not really looking to the future yet, so my mind's not focused on whether I'm going back or not going back,
said McManus.
I'm really enjoying my time here, and I'm just going to focus on the games that are left between now and the end of the season, then I'll turn my attention to the future.
We'll see how it goes, but nothing's been decided.
Source: Northern Echo
Source: Northern Echo