The Colombian midfielder has been linked with a move to Villa Park for the majority of the summer for a fee of around £4m.
Lambert told reporters: "We'll have to see what happens over the next few days.
"I'm never quite sure until he's signed on the dotted line. We've got until the end of August."
The defensive midfielder, who made 30 appearances for the La Liga side, has spent the majority of his career playing in France for Valenciennes.
Sanchez has a release clause of £4m in his contract, which was set after joining Elche from Valenciennes in 2013.