The most interesting thing about those quotes from the manager is that they may reveal a recognition that Rossi and to a lesser extent Pique were lost to the club because they were not prepared to wait to be integrated in to the first team on the timescale that the club would have preferred.
I suspect that Sir Alex probably believes that Pogba is too young to play a role in the first team and that he would prefer foreign born players to follow a similar development strategy to their British counter-parts, but I can certainly understand the logic of at least involving them in the first team squad and possibly even some games in order to convince them that they are going to be an important player in the future.
Having said that, physical development is almost as important as maturity and experience when introducing young players in to the team, so it is entirely possible that the manager simply believes that Pogba is physically advanced in comparison to many players of the same age, despite the fact that there is clearly a lot of physical development yet to be realized.
It's also important to note that even if Pogba moves up to the first team squad, it says very little about his immediate role in first team affairs. Of course, if he is outstanding in training and particularly in limited appearances on pitch, then the circumstances may dictate an accelerated integration in to the team. At this point, however, it's not credible to suggest that an 18 year old is in any way analogous to a 27 year old international who has won the Champions League and appeared in a World Cup Final.
The often repeated phrase that 'if you're good enough, you're old enough' is in one sense true, but it's also incomplete and therefore misleading. Pogba's talent is not really in question, but it's important for both the player and the club that he is allowed to develop as naturally as possible. Different aspects of a footballers game develop at different rates and it is possible to damage a players development and therefore the expression of that talent on the pitch by overlooking the mental and physical aspects of the game.
I can certainly understand the eagerness to see an obviously talented player appearing in first team games. The appealing logic of that position appears to be that it will accelerate the development process and therefore benefit the team. But it's important to recognize the inherently selfish nature of that position.
On the one hand, there is the desire to see that player in the first team. And on the other, there is a focus on how that player can benefit the team. Conspicuous by its absence is an emphasis on what is most beneficial for the player, specifically.
That is why I believe that any discussion about whether a young player is ready to be integrated in to the first team should always start from a position of whether it would be in the best interests of that players long term development.