There are many examples where Rafa has clearly just signed players on the cheap to balance the squad. Pellegrino, Josemi, Kromkamp, Nunez, Gonzalez, Barragan, Itandje, Leto,... are examples of that. It's only natural that he'd be prepared to move them on quickly.
In particular in his first seasons, Rafa had an immense job to do to shape up the balance. If you look at today's squad, it's "cleaner". You can always improve, but my point here is not to evaluate if player A has been a success or not.
My point is to try and explain that Rafa has cleaned up the squad, so it's now a lot easier to see where we need to improve. And we can focus on specific areas as a result. Or, if you wish, spend more on a selected few.
The whole point with this is to have a tight squad. Man U have been in that situation for years. Makes it so much easier to focus the spendings. Which of course, is why Man U only need to make a couple of signings while we've needed to "waste" money on more. Naturally, with a tight squad and squad balance, you'll find it a lot easier to have patience with promising youngsters and you can give them time.
It all comes back to squad balance. Needs to be taken into consideration when you evaluate how successful Rafa has been in the transfer market.