England midfielder Lampard suffered a muscular injury in Sunday's victory over Liverpool and defender Ivanovic damaged his knee in the same game.
Mourinho confirmed after the New Year's Day victory at Southampton that neither player will feature again until the end of January.
Chelsea manager Mourinho is in no rush to strengthen his squad now that the transfer window has opened.
The London club are reportedly keen to sign a striker, but Mourinho does not envisage being busy in the transfer market this month.
He said: "The window is open and when it's open anything can happen but we don't think a lot is going to happen for us.
"If somebody really wants to leave probably we have to bring in somebody but we are going to do things in a very calm way.
"I know what I'm building and what we are building at the club. Next summer will be a moment to make a couple of clinical buys and improve our team in that direction.
"We work and improve and try to bring the young players to a different level, and we are doing that quite successfully. We are not candidates to win the league, but we are always candidates to win the next match."
Chelsea's 3-0 win at St Mary's leaves them in third place in the Premier League, just two points behind leaders Arsenal.
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