Arca left Boro's Rockliffe Park training ground with his personal possessions on Thursday afternoon after talks over a new deal broke down.
The Argentinian became a free agent at the start of the summer, but despite Tony Mowbray expressing a desire to retain his services, the two parties were initially unable to reach an agreement.
As a result, Arca was approached by a number of Championship clubs on Friday morning, including Boro's opponents this weekend, Leeds United, and had been expecting to talk terms with some of them later today.
However, he was persuaded to attend the Riverside Stadium on Saturday, and met Mowbray again after watching Boro kick off their Championship campaign with a 2-2 draw with Portsmouth.
A second round of talks proved much more productive, and Arca agreed personal terms late on Saturday night.
An issue over an outstanding payment from the midfielder's previous contract is still to be resolved, but provided there are no unforeseen problems, a deal should be concluded shortly.
The news will be a major boost to Mowbray, who was forced to play full-back Joe Bennett as an out-of-position midfielder at the weekend because of a lack of options on the left-hand side.
Arca will also be delighted at the developments, as his initial decision to return to training with Boro stemmed from a desire to remain in the North-East.
The South American is settled in the region and has already rebuffed an opportunity to return to his homeland with Boca Juniors.
Source: Northern Echo
Source: Northern Echo