Lukaku missed victory at Everton due to a head injury.
Before kickoff, Mourinho told Sky Sports: "I would say a week [out for Lukaku]. It's nothing really important - just a little protection, but no problem."
Asked whether the Reds will need to adapt their gameplan in the Belgium striker's absence, he said: "Of course. If we don't have a player like Lukaku or a normal replacement like Zlatan, we have to play with the qualities we have, so we have to forget that we have to play without a target man because we don't have [one].
"We have what we have. We trust the players," he added. "The players are giving everything like they have done from the first minute of the season. No Lukaku and no Ibrahimovic means we have no no.9 as you normally call a target man that we usually play [with], so we have to change."