The 21-year-old will put the finishing touches to his move to Craven Cottage on Monday after undergoing a medical.
Sanchez sees the Republic of Ireland B international as one for the future and will send him back to Cork on loan until the end of the year.
"O'Donovan's a young lad from Cork who we've agreed a fee for and he's having his medical on Monday," Sanchez said.
"He will then be going back on loan to Cork until the end of their season and then we'll have a look at him.
"He's done very well for Cork but he's definitely a young lad for the future rather than an immediate starting player."