Tuesday 14 March 2017 13:44, UK
Stewart Downing and Middlesbrough boss Aitor Karanka were involved in a training ground row last week, Sky sources understand.
Downing is understood to have questioned why he was left out of Middlesbrough's squad for the 2-0 Premier League loss at Stoke the previous weekend.
The former England winger is believed to be frustrated by his lack of first-team chances this season, having started 13 matches for the Teessiders this term.
Downing and January signing Patrick Bamford were both left out of Boro's 18-man matchday squad for Saturday's 2-0 FA Cup quarter-final defeat to Manchester City.
During his post-match press conference, Boro boss Karanka - who also benched Alvaro Negredo and Gaston Ramirez against City - appeared to question the desire of some of his players some of his players.
He said: "Now in the difficult moments I need 18 fighters in my team.
"Where the players show me that they are ready to play and they are fighting and they are going to fight for the club is on the training ground every day.
"I am with them every day and on Friday, I will always choose at least 18 fighters."
Middlesbrough have failed to win any of their last 10 Premier League matches and are second from bottom in the table.