Tavares was on Teesside yesterday putting the finishing touches to a loan move from German side Hamburg.
Hamburg officials gave the 27-year-old permission to fly to England on Thursday, and Boro have agreed a loan deal for the central midfielder.
Tavares will initially sign until January, but there is an option to extend the agreement until the end of the season if all parties agree.
Boss Gordon Strachan had hoped to include the Senegal international in his squad for today's trip to Oakwell, but the process of confirming the player's international registration proved more difficult than expected.
Boro officials were in regular contact with German FA officials yesterday, and Strachan remains hopeful Tavares will be given the goahead to take his place in the squad in South Yorkshire.
He will not feature in the starting line-up whatever happens, but could help bolster a bench that will contain a number of young players.