Aitor Karanka says Boro will be without two players, but two more are in contention to return for Saturday's Premier League tie at Watford.
Antonio Barragan limped off in Boro's 0-0 draw with Leicester City, while Gaston Ramirez was not included in the Teessiders' squad for the FA Cup win against Sheffield Wednesday.
Victor Valdes has not featured since Boro's New Year's Eve trip to Manchester United, while Viktor Fischer has been absent since mid-December with a knee problem.
"Valdes is available, and Fischer is back in training with the team so he is fit to play," Karanka said.
"Antonio Barragan and Gaston Ramirez are the players who are out.
"Gaston has a problem with his knee and we thought it was going to be a few days but know we will wait one or two weeks and see what is happening.
"We hope it is not serious but you can't rush this type of injury."
Dani Ayala will also miss out as he serves the first of a three-match suspension.
"It's another defender who we have lost, but I think we have enough cover.
"You can understand Dani's frustration.
"Sometimes you want to show how much you want to play, but he will learn from the mistake."