The 23-year-old has failed to break into the starting XI at Goodison Park, playing just eight Premier League games since joining the Toffees from Malaga for £5.2 million ($6.8m) in July 2017.
Sandro is expected to travel to San Sebastian on Thursday to sign a contract with Sociedad, who will not have a purchase option in their loan agreement with Everton.
Sociedad president Jokin Aperribay confirmed that the club were close to sealing the transfer, with the two teams already in agreement over a one-year loan.
"We are close, quite close, there is agreement with him and with Everton," Aperribay told reporters.
"Today they play a cup match and can come tomorrow if all goes well. If it is done, it will be done tomorrow."
Aperribay also indicated that another signing is unlikely, but if Sociedad sign a second player before the transfer window closes this week, then it could be someone to strengthen their midfield.
"Surely no one else will come," he continued. "Only Sandro and maybe a central player."
The forward has a contract at Goodison Park until 2021 and has been praised by Everton manager Marco Silva, who was impressed by his attitude and desire to turn things around.
“I don't like to talk about these situations all the time," Silva told the Liverpool Echo last month.
“I spoke with Sandro two weeks, three weeks ago, I was clear with him, he was clear with me. He did a good pre-season, he worked hard and took his position in our squad. He is a good professional, a very good professional.
“Of course, when he didn't perform like he wanted to during his first six months with the club for him it is clear the competition will be tough – but not just for him but the others as well."
Sandro last scored a league goal in 2016-17 while at Malaga, so will be hoping for an improvement on his last stint in La Liga, where he failed to find the net in 13 league and five cup appearances for Sevilla.