United currently lead the Premier League title race by two points with a game in hand on their main rivals.
They have also made it through to the last 16 of the UEFA Champions League and the FA Cup, while they will face Tottenham in the Carling Cup final at the start of March.
Assistant boss Phelan believes United have to approach their quest for an unprecedented quadruple in a sensible manner.
"Everybody wants the four trophies but it has never been done," said Phelan.
Finishing line"It is obviously difficult and we are just happy to be in it at the moment, getting nearer to the finishing line in every competition.
"The club has been built to sustain challenges in four competitions, so now we will look to the FA Cup and try to pick the right team for Sunday."
United have been in supreme form of late but Phelan is refusing to take anything for granted, with 14 league games remaining.
"It is a race, that is the thing," he said.
"Where the challenge comes from doesn't really matter.
"If we train well and play well, pick the right team and the players do their jobs we have a good chance to capitalise."