The 20-year-old, currently in his second loan spell at npower Championship Hull under former United defender Steve Bruce, will start Wednesday night's friendly against Greece at the Aviva Stadium.
O'Shea said: "He got a few more chances then when he went back in, in pre-season and a couple of the League Cup matches.
"It's one of those things. Look, Robbie is a very intelligent lad, he is under no illusions that he is facing a very difficult task.
"But he has got a lot of ability and he has got the talent to back it up, so if he did get a chance to get a run in the Manchester United team, I am sure he would take it with both hands.
"But as I said, he's got a lot of competition."