A few thoughts...
1. As with any player, they have to force their way into the managers thinking with their displays at Reserve team level and in training against other options the manager has. Mejbri normally plays as an attacking midfielder so that means he is up against Fernandes, Pogba and Lingard for a place in the team. Solskjaer sees him in training every day so is best positioned to make that call.
2. You can test their potential by giving young players some cameo's at the end of games where the result is probably a given. The last 15 mins against Leeds was a good example. However, given our pre-season there were other first team players and new signings that also needed some match time. I would expect Mejbri to get some more opportunities this season.
3. Of the 120+ loans that United have used over the last 20 years statistics have proved that 95% don't work out for a variety of reasons. I can only think of O'Shea, Welbeck, Evans, Lingard, Henderson and maybe Cleverley, where they came back as improved players and went on to play regularly in the first team. For the others it confirmed their 'level' or had no discernible impact. In fact in most cases, the player came back to United with an acceptance that they probably wouldn't make it and needed to move on. So whilst a loan might help Mejbri there are no guarantees.
4. There is a massive difference between playing in most European leagues at 17/18 versus doing so in the English top division. Whilst I don't watch every other league every week, the physicality and culture in England is so different. Even experienced internationals coming here as senior players comment on how it takes them a season to adapt properly. Someone mentioned wide players and they tend to get more chances as they are little more protected. There might be some truth in that as Ferguson proved with Darren Fletcher so perhaps Solskjaer has an option there. On the other side we still have Sancho, Rashford, Greenwood, Martial, Elanga who are genuine wide players with pace.
5. You can't play everyone, and Mejbri may just have to bide his time.