Sir Alex Ferguson’s side have been strengthened by the arrivals of Phil Jones, David de Gea and Ashley Young, but United’s assistant admits there are more teams who could challenge for the title.
"Manchester United always have to play well," said Phelan.
"We always have to be in contention to win things. It will be no different next season. If anything it will be even harder.
"We worked very hard to achieve what we have in the past. Next season will be very difficult because there are a lot more threats now than there has ever been before."
Manchester United face rivals City at Wembley next weekend in the Community Shield.