The Red Devils suffered their worst-ever Premier League campaign last season, finishing 15th with only 11 wins from 38 matches.
Rangnick, who managed Manchester United from December 2021 to May 2022, famously stated that the club needed "open-heart surgery" to restore its former glory.
"If I were the owner/sporting director, I would only try to sign and invest in young players because no matter how you look at it, it only makes sense,” Rangnick told Sport.
"Does it make sense to spend a transfer fee of 30/40/50 million for a 30/28 year old? Let’s make that calculation. You sign a 30/28 year old, give him a five-year contract.
"You spend on average 15m/year on wages, times five, that’s 75m. The total is about 110/120m, then there are the agent fees. The whole deal is about 150m. For what?
“You’ll never get that money back. If you’re unlucky, in the last years of his contract, the player doesn’t even play anymore. The only thing you do if you do that is you invest in a possibly greater amount of success in the first few years. This is like a bet. I would never allow to do that, allow that to happen."