The 29-year-old winger signed a short-term deal at the Riverside in January but has missed the last four games with the issue.
Boro will hold contract talks with players at the end of the season, with Mendez-Laing, Marvin Johnson and goalkeeper Jordan Archer all set to see their current deals expire.
When asked by the Mail if Mendez-Laing could feature in Boro’s final two league games against Luton and Wycombe, Warnock replied: “I doubt it. He is getting better but it’s touch and go and his contract is up as well.
“From his point of view he has to get himself right for pre-season so I can’t expect him to come back for one game if he’s not right.”
Boro are likely to have the same squad available to face Luton as they did against Sheffield Wednesday, while Djed Spence has returned to training.