I see more positive sides when the kids start and play in AHL until they are really ready.
First thing is that pre-season camp can be deceiving, many veterans are still shaking off the summer rust and since their jobs are secured, i don't believe that are playing like their lives depend on it. After all pre-season games are meaningless to them, they ain't looking to get hurt right before the season starts. And the other teams are playing their kids and AHL depth as well, some of those kids are still far away from NHL, so naturally our more NHL ready kids will look a whole lot better.
Pre-season camp is also quite short and 19-21 year tend to gas out after the mid point of the regular season, and that's not really good when we are hunting for a playoff spot. Quite a big risk to throw in 3-4 rookies and if/when most of them gets gassed out, who's gonna replace them? AHL depth, guys on waivers or are we going to need spent assets to acquire players? Or are we just going to call it quits and better luck next season?
Second thing is depth, injuries happen every season and who's going to replace them? AHL depth? Kids who ain't ready? Or 4th liners? It is better to start the NHL ready kids in AHL, let them build confidence and when the injuries happen, then they get called up. And when that happens, then we'll see if they are really ready. Intensity will be on the whole other level during the regular season than in the pre-season. AHL veterans stays down to help the AHL team win.
If all the NHL ready kids would be up and the injury wave happens, then the AHL vets would get called up. And that would leave the poorer AHL players and bunch of ECHL players with 20-year old rookies leading the AHL team. Then both teams would be losing, since we all know that career AHL players ain't a long term solution in NHL.
Third thing is responsibility, in AHL the most NHL ready kids will play in a big role. Top-6 on 5on5, PP1/PP2, PK1/PK2, first OT units. There they are leading the team, carrying the responsibility of their teams success. There they are not complement pieces on smaller minutes.
I like the approach what Wings have, prospects are given all the necessary time and they really have to work their way in. Some top prospects has never had to work that hard before, because they have been so talented. Their natural talent has carried them this far, so some of them haven't been really challenged before.
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But answer to the OP question.
For AlJo, since he is up, it's all up to him now. I don't see him having problems to out play Määttä and Holl. I think Gustafsson has a different role so AlJo is not really competing against him.
Motte is a 4th liner and PK-specialist, i see Mazur having much bigger potential than just a 4th liner and PK-specialist. Mazur should most definitely start in GR. Big role there 1st line, PP1, PK1/2.