Boro were keen on Standard Liege centre-back Onyewu two weeks ago, but his poor performances at the World Cup and his reluctance to commit to a move have convinced Boro boss Gareth Southgate to pull the plug on the player's prospective move to the Riverside Stadium.
Senegalese international Diawara has been in fine form in Ligue 1 for Sochaux, and both Bolton and Middlesbrough have been tracking the player.
Weekend reports suggest that the player's agent Lee Anderson has indicated Boro lead the race for the player's signature, and could complete his £2.4million move in the next fortnight.
Meanwhile, Bolton's bid for Marseille stopper Abdoulaye Meite has stalled.
A row about agents' payments has put the £1m deal for the Marseille star in jeopardy.