The Scotland international started on the bench in Sunday's 2-1 loss to Sevilla on Sunday.
Former Red Devils star Berbatov told Betfair: "The defeat showed that United need to strengthen their squad this summer. They need a centre-back for sure.
"But United need more depth, Ole made his first substitution very late [against Sevilla] when things weren't going well.
"It does show he trusts his first 11 a lot, but maybe it shows he doesn't trust his bench so much. There will be lots of games next season, and they need to have the squad to be ready.
"I find McTominay's situation really strange. He is fit and ready, he got his contract extension, but he doesn't play. Is it because Matic and Fred play really well? I am not sure, but I like him a lot, so hopefully he can pick up the pace next season and get back into the team."