Ibrahimovic was released from his Manchester United contract three months early last week having made just seven appearances this season.
The Swede was always expected to leave Old Trafford upon the expiration of his contract in the summer, but Mourinho has no plans to bring in a replacement.
“We have Lukaku and Marcus as No 9s,” the United manager said on Friday. “Of course Marcus can play right and left but Marcus is a striker.
“So, Lukaku and Rashford, Alexis [Sanchez] can play as a striker, [Anthony] Martial can play as a striker.
“Without having a replica of Lukaku, we have three more players who can play as a No 9.”
Mourinho added: “If we don't have Lukaku for different reasons, we can change a system and play with a two.
“We can give more mobility to, for example, Marcus and Martial together, Marcus and Alexis together, so we are not going to go for another No 9.”
United are expected to strengthen in a number of positions once the transfer window re-opens, most likely left back, central defence and central midfield.
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