We might not know, as fans, but you'd like to think that the manager will. There's a reason Kjartansson hasn't been given a run out whilst the rest of our midfield is struggling, and a reason he wasn't in the European squad. He's only 19, and there isn't a huge rush if he isn't ready. I don't believe that Marshall or Kjartansson should be starting for us, but we need to make room in the squad for them to get minutes (or send Kjartansson on loan if he's not there yet). It's still a big step up from Arbroath to Aberdeen, and it wouldn't surprise me if Marshall didn't have the required athleticism to make it at the higher level.
The biggest problem that both would face is that JT hasn't got the slightest clue what our best midfield setup is anyway. Shoehorning Marshall into that horrific 3-4-3 at right forward would do the guy no favours, and he doesn't look to have the legs to play box to box. Which is our biggest problem with these two young lads. The role we are looking for is a guy that is athletic and can cover lots of ground. They'll suffer if we don't get that player.