The central defender quit Valencia at the end of his contract on June 30 and is now a free agent.
Garay, 33, is in the final stages of recovering from a serious injury which has kept him out of action since February.
He is determined to continue his career at the highest level and sees a return to Benfica as an excellent opportunity to re-find his former level.
Although he could arrive at the Lisbon club on a free transfer, talks have not yet concluded.
The Argentine international earns a very high salary, more than €2.5m-per-season, and doesn’t want to accept much less.
The player’s wage demands could be the main obstacle in Benfica getting a deal over the line.
After strong performances for Benfica, several European clubs wanted to sign him, with Zenit winning the race.
Garay has won 32 caps for Argentina since 2007, with his most recent coming in 2015.