Manchester City's John Stones has been regularly sidelined with a string of hip injuries this season.
The holders take on RB Leipzig at Etihad Stadium on Tuesday in their fifth Group G game, having won four out of four so far.
They and Leipzig have already clinched spots in the knockout stage, but first place in the pool remains up for grabs.
"We have to win or draw," said Guardiola.
"The first step is done for both sides so congratulations to RB Leipzig to qualify for the next round in February.
"It is important to finish first. We believe that when we play the last 16, [having] the second leg at home is not decisive but an advantage and we have to take it."
City beat Leipzig 3-1 on matchday two. That result and City's three-point cushion over Leipzig makes the permutations as straightforward as Guardiola laid them out.
Avoiding defeat will do the job and eliminate the potential danger of City finishing second and having to face the winner of another group in the last 16.
City have every reason to be optimistic. Leipzig are fifth in the German Bundesliga, 11 points behind league leaders Bayer Leverkusen, while City have not lost at home since November 2022.
Guardiola delivered a pessimistic message about a number of fitness issues affecting his squad.
Jack Grealish, Mateo Kovacic, Sergio Gomez and Matheus Nunes are doubts.
Kevin de Bruyne is a long-term absentee, but there is positive news in the case of John Stones.
Stones suffered an injury in City's win over Young Boys on November 7 but is "really close to coming back", Guardiola said.
Grealish missed Saturday's 1-1 draw with Liverpool through illness.
"I'm not optimistic about some people coming back but we will see," Guardiola said.
"I think it will be the same people who played against Liverpool. John Stones is getting better. He's not for playing but maybe a few minutes."
-- Courtesy of BBC Sport