The United duo have been thrown in at the deep end this season following the departure of Nemanja Vidic and Rio Ferdinand in the summer and Louis van Gaal's failure to sign a centre-back.
The Dutchman spent £159m in the summer after taking the hot seat at Old Trafford.
He brought in the likes of Angel Di Maria, Radamel Falcao (on loan), Luke Shaw, Ander Herrera and only signed Marcos Rojo who could play at centre-back.
United are 13 points off league leaders Chelsea and already out of the title race.
But McGrath isn't sure there is anyone in the market worth buying and the better plan would be to develop the young defenders.
"Old Trafford can be a horrible, unforgiving place to learn," McGrath told Mail Online.
"When I see Tyler Blackett, he looks so smooth he could become a real gem but it’s whether they give him the leeway to develop.
"Everyone wants results now and pressure comes with that, which makes you nervous and leads to mistakes. I made countless mistakes but I was allowed to grow.
"Vincent Kompany is a great leader, Mats Hummels is similar but United aren’t the only ones with money any more. It may be better to nurture Phil Jones, McNair, Blackett and Smalling because they can grow into better than what is in the market."
