We already have Sanchez and Mata on our books. The striker (Lukaku) has many years of experience too.
If Mourinho wants it at any costs, then we simply have to do it, otherwise the manager has to get the sack. Watching a young player go from strength to strength fascinates me more than seeing a prime player slowly regress. But of course winning is a must and someone is in charge and decides.
The next question would be whether or not Martial will be here next season. Replacing Martial with Willian and only having Rashford as a young gun around would be a tad sad, IMO.
Lukaku, Sanchez and Mata are all very experienced. Young and Valencia can also play further up the pitch. At the moment we have two young players who we'll develop (Martial and Rashford). Selling one and replacing him with a player who's nearing 30 years of age would leave us with only one talent. As stated above, i can come to terms with it, but personally i wouldn't do it.
City's Sane didn't take too long to hit the ground running, he's only going from strength to strength ever since. Is Willian who's never hit double digits in goals/assists in the Prem the right answer in closing in on the gap? Ability wise Bale would be a suitable so-called stop gap, IMO. We need someone who can become a real star very soon, if it means paying double the costs for what a Willian would be demanding, i'd do it. Mourinho's head is on the line next season, maybe he'll survive after putting in a genuine title challenge, but Willian won't get any better in the next 2-3 years, whereas Sane/Sterling/Jesus are just starting to take off. We need to look at the big picture here.