Well, he was around first team similar like Garner today for example, but hardly really part of the team. At least not until the last few weeks when he got a few chances both at RB (against Arsenal when he was very good) and DM. I am talking about his first season with Mourinho. In his second, he did not play at all, until he left to Villa in January and got injured, playing just a few games again more as RB. His 3rd season was season long loan to Villa, when he was integral part of the team (when not injured), getting promotion. He was very good, especially for his first full, 90 minutes per game, senior season. He showed lot of potential, had a few MOTM performances and again he had injury. So, this first season under Ole and his 4th, is his first in Man Utd when he is real option. And I agree with you, he did not show much this year, but he did not play much, again cause of injuries. He was pretty good in preseason, he was OK in all games he played, but not like world class if you expected something like that. I would not agree that he cost us points against Arsenal, he was pretty good in that game. Mistake for goal is just partly his, but Lindelof was slow and positioned wrong, while Maguire also too slow and put Axel on the pressure. But, certainly that was mistake, and he will made a lot of more in the future. In EL I think he was more than OK.
But, this is not thread for Axel, so maybe we are out of order here. Regarding Mengi, I think he is good, but lets see how he will make it on loan (if he goes) against men. In U23 and U18 he is dominant, but that is big step. Lets hope he will make it like Axel to become an option, and that he won't have injury problems, so can develop faster.