United are struggling this season with a 2-1 defeat to Sunderland on Saturday severely denting their hopes of a top-four finish.
“It’s difficult for them as the quality in the Premier League is increasing,” O’Shea said.
“Champions League qualification is the minimum target for them – that’s a given beforehand, when you look at the squads and the budgets and what is spent.
“And not just Man United, the top five and six spend big to get into the Champions League too.
“That is where the rewards are and that is what attracts bigger and better players.
“That is the challenge they have and they are having to adjust to.”