United were thumped 4-0 by the Toffees at Goodison Park.
Osman, who made more than 400 appearances for Everton, believes there should be a huge overhaul – except for one player.
Speaking on Stadium Astro, the Everton legend said: “It's so difficult.
“United are underperforming, they're under-par, they're not anywhere near the team we expect."
Osman continued: “Personally, from what I've seen over the last four or five weeks, Rashford would be the only guy I'd be determined to keep."