The west London club have already tabled two bids for the England forward and have not ruled out making a third, despite new United boss David Moyes's steadfast refusal to sell. (Agencies)
"We have Plans B and C," Mourinho said ahead of Wednesday's home game against Aston Villa, which they won 2-1 thanks to a second-half header from Serbian Branislav Ivanovic.
"Don't ask me names because it's difficult to speak about players from other clubs. We have the conditions to try to improve our team by bringing in one more player and we are going to try that till the end," Mourinho said.
The Blues are keen to add to their striking options of Fernando Torres, Romelu Lukaku and Demba Ba but United do not want to sell to a Premier League title challenger despite the form of Robin van Persie and Danny Welbeck, who each scored twice in an opening 4-1 win at Swansea City on Saturday.
Monday's match between two of the Premier League's biggest rivals sees David Moyes in charge of United at home in the league for the first time since replacing Alex Ferguson, a man whose absence the returning Mourinho said would be strange.
"It will be difficult for me, and I think all managers, to go to Old Trafford and play against Manchester United without this mythical figure waiting for us," he said.
The match (kickoff 1900 GMT) is part of another busy start to the season for Europa League holders Chelsea, who take on European champions Bayern Munich for the Super Cup in Prague on Friday, a repeat of the 2012 Champions League final.
It is also the first in a tough run of fixtures for former Everton boss Moyes' side, who also play Liverpool and Manchester City in coming weeks, and the Scot will be desperate to make an early mark in front of his new fans at Old Trafford.
The west London club have already tabled two bids for the England forward and have not ruled out making a third, despite new United boss David Moyes's steadfast refusal to sell.