Sandro has made little impact at Goodison Park since joining the club last summer, with just one goal coming in 15 appearances in all competitions.
The Spaniard spent the second half of the campaign at Sevilla, where he failed to net in 18 outings, but it appears that the 23-year-old has been given another opportunity in his homeland.
Club president Jokin Aperribay told reporters: "We are close, quite close, there is agreement with him and with Everton. Today they play a cup match and can come tomorrow if all goes well. If it is done, it will be done tomorrow."
Sociedad sit in seventh place in the table after the opening two weekends in La Liga.