The 22-year-old midfielder completed his move to the Riverside from Argentine club Banfield this week following his involvement at the Tokyo Olympics.
Boro hope Payero will get international clearance on Friday, which would make him available for Sunday’s Championship opener against Fulham.
When asked about Payero, Warnock said: “We still haven’t got international clearance. We still need international clearance which we hope we can get this morning.
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“He’s a lad we’ve looked at for quite a long time and I’ve spoken to him a couple of times, he seems like a really nice lad and I think he’ll fit in well.”
Payero could be in the squad for the Fulham match but is still getting back up to speed following his involvement at the Olympics.
When asked what Payero can bring to the side, Warnock added: “I think it’s just his enthusiasm, he’s got a great touch, both feet and I think he just gives us a bit more quality in that area.
“We have a lot of lads who are very similar and will do a job but sometimes you need somebody just to unlock the door and I think Martin has got that in his locker.”
Sammy Ameobi will miss the match with a knee injury, while Marc Bola is back in training after isolating.
Marcus Tavernier has been sidelined with a hamstring issue in recent weeks and will be assessed ahead of the match, while Hayden Coulson remains out.
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