Lukaku was stretchered off during Manchester United’s 0-0 draw with Southampton on Saturday evening.
The Belgian went down after clashing heads with Wesley Hoedt and was treated on the pitch by United’s medical staff for almost four minutes.
Lukaku is due to miss United’s next two matches, according to Mourinho. They face Everton at Goodison Park on New Year’s Day and then host Derby County in the FA Cup four days later.
“When you see a player leave the pitch like he did, it is usually two games at least,” Mourinho said.
United thought they had a replacement for Lukaku in Ibrahimovic, but he is expected to be out a for a month after Mourinho revealed the Swede is facing ‘a big problem’ as he continues to suffer from knee surgery earlier this year.
He has “massive problem” said Mourinho, “He is a 37-year-old with a bad knee.”
Mourinho will be forced to use Marcus Rashford and Anthony Martial as his main strikers now as he tries to get a team that looks devoid of confidence since the defeat at home to Manchester City.
In the six games since the derby defeat, United have won only two and are winless in their last four matches in all competitions and have slipped to third in the table.
Mourinho needs to get United back on track or they will find themselves involved with Liverpool, Spurs and Arsenal for the last Champions League places.