This would be a good plan for the Red Devils honestly. Let's check out the current alternatives in the position.
Luke Shaw... He has suffered big injuries especially in the recent years. This just finished him. The team can't rely on him in the long term anymore. Otherwise he was a quite decent LB. Injuries hit him hard.
Tyrell Malacia... He is another alternative who is much younger. However he couldn't even play in last two seasons. He suffered a huge injury period which lasted nearly a year and a half. I don't think they would like to trust him as the main LB either.
Playing Diogo Dalot or Noussair Mazraoui at left-back is unlikely to be an option for Ruben Amorim, meaning that if Man United does sign a new left-back, one of Dalot and Mazraoui will be benched. There are no specific names, so it will definitely be a lot of speculation here. Alphonso Davies is one of them, and he is currently being linked with Man United, but I think Man United really has to fight hard if they want Davies because besides having to prepare large funds, Man United has to compete with Real Madrid which also seems interested in Alphonso Davies.