Ferguson knew that. Seemingly Mourinho knows that. Sadly fanboys don't seem to grasp it.
Of United's 'world class' signings, in the Premier League era, how many have succeeded? Veron didn't, ADM and Falcao didn't, Pogba likely will but hasn't performed like you'd expect him to initially. Mata can't establish a starting berth and Mkhi has blown more cold than hot. Van Der Sar's stock had fallen by the time he arrived, Van Persie's success was isolated to one season. Berbatov never really fit in.....id say only Tevez arrived and left a great player.
United's ethos, much like Juve's and Bayern's, was to buy up the best domestic talent from within the league, players on the cusp of being considered top-level, and cultivating their development towards that status. Keane fits that criteria to a T, as does Dier. Both would be great additions and have are of the correct age to be part of a dynasty.