We can say that Mason Mount is a promising player because he is 24 years old. Chelsea will want to keep him first, as he is one of the players who benefit Chelsea. If he cannot keep it in the team, they will not approve his transfer before the transfer fee they will earn from him is high. If Manchester United makes this transfer, the burden on Rashford may be lessened.
The thing is Chelsea feels pained selling one of their talented players to a rival club and have already had 2 bids from United turned down. They cannot keep Mount because he has made his intensions known to the club that he does not want to stay and he has just a year left in his contract. If Chelsea doesn't sell him now they risk losing him for free next season so they've got themselves in a very tight corner. It's rumoured that Chelsea's camp is working on how to reach a resolution concerning the situation.
Concerning lifting the burden on Rashford... I hope Mount hits the ground running quickly. The red devils will also be welcoming Mason Greenwood to the team which is a plus to their strike force.