January 18, 2017 0808 GMT
Ibrahimovic missed Tuesday night's 2-0 win over Hull City in the EFL Cup semifinal first-leg through illness, having been expected to lineup, skysports.com reported.
But Mourinho does not anticipate the former Sweden international being absent for much longer when Liverpool visits Old Trafford as part of Merseyside v Manchester.
Ibrahimovic has been in superb form for United, scoring 12 goals since the start of November, and if available would be well rested for the clash with Jurgen Klopp's side.
He was an unused substitute when United moved effortlessly into the fourth round of the FA Cup with a 4-0 win over Reading last Saturday.
"Zlatan is ill so I think no problem, I think he will be fine," Mourinho said when asked about his availability for Sunday's game.
Defender Marcos Rojo suffered an injury in the win over Reading, and also missed the Hull clash, with Mourinho saying he remains doubtful for the weekend.
"Marcos has a muscular problem, not big. I'm not saying he's not ready to play, I'm saying that I have my doubts," he added.