The Premier League is out of reach for Manchester United this season, with Xavi warning Jose Mourinho and Co. to lower their expectations.
United's title hopes took a blow on Sunday, humiliated 4-0 by Chelsea in Mourinho's return to Stamford Bridge.
"The domestic competitions have to be everything for United and Mourinho this season," Xavi said in quotes attributed to the Mirror.
"I don't think this season they can win the Premier League. These competitions are their hope for a trophy.
"Once Mourinho has had another transfer window, maybe they'll be ready, but for now they must secure a finish in a Champions League spot - that's where United belong."
Xavi added: "Sometimes, when things don't go well for Mourinho, he wants war not football - like when he was at [Real] Madrid - and I hope we don't see that again in England."
Pep Guardiola's City head to Old Trafford in the midst of a five-game winless run in all competitions.
"Both coaches are going to want to win this game," said Xavi. "I don't believe this is a competition they will not take seriously.
"Pep will not be worrying, that's not his way - but he will not have been happy with recent results - and a good win against the biggest rivals could put them on a good run again.
"Pep will also want to win his first competition with City, that will be important for him. Even with City dropping some points in the league, I still think under Pep they'll be champions this season."
