The Red Devils are linked with a move for the Borussia Dortmund winger this summer.
With days to go in the transfer window, it is unclear if a move is possible.
And McClaren thinks the squad may need strengthening elsewhere.
"Sancho is a luxury they don't necessarily need," the former United assistant and England manager told talkSPORT.
"You're only talking £10m or £20m more in £100m to go out and get him. Go and get the boy – you're Manchester United. Show your clout.
"But I do think you might as well spend that money on decent defenders."