Press Association Sport understands the Spaniard left the training ground unexpectedly on Friday following a meeting, and his dramatic departure triggered a series of rumours.
Boro, who travel to Charlton in the Sky Bet Championship on Sunday, went into the weekend sitting in second place in the table, four points behind leaders Burnley with a game in hand and two points clear of the chasing pack.
However, they have won just three of their last 10 games and taken 12 points from a possible 30 to surrender what was once a commanding lead.
There is also an ongoing debate over Karanka's use of £9million January signing Jordan Rhodes, who has started just four times in eight appearances and scored a single goal since his arrival from Blackburn.
Source : PA
Source: PA