Middlesbrough head coach Aitor Karanka is considering his future at the club.
The 42-year-old Spaniard, who took charge at the Riverside in November 2013, left Boro's training ground unexpectedly on Friday after a row.
Boro, who travel to Charlton on Sunday, are second in the Championship, four points behind leaders Burnley with a game in hand.
The club confirmed Karanka did not take training on Saturday and assistant Steve Agnew takes charge at Charlton.
"We appreciate supporters will want to know further details, but at this time the club cannot make any further comment and is solely focused on Sunday's game at The Valley," the club statement added.
Middlesbrough led the table by six points on 12 January, but they have won just three of their past 10 games.