A 2-1 Manchester derby win at Old Trafford on Sunday - marred by an altercation in the tunnel after the match - extended City's already massive lead from their rivals even further.
City have won 14 Premier League games in a row, setting a new record for consecutive victories in a single season, despite injuries to Benjamin Mendy, John Stones and Vincent Kompany.
With City's defensive resources stretched despite heavy spending in the previous transfer window, Guardiola is prepared to bring in more new players in January.
"We are thinking about it," Guardiola told reporters ahead of Wednesday's visit to relegation-threatened Swansea City.
"In that department [defence] we have few, few, few, few players and to handle all competitions we are in trouble.
"With the club, with [director of football] Txiki [Begiristain], we are going to see what we do in January."
Injuries to Stones and Kompany have opened the door for Eliaquim Mangala to come back into Guardiola's thinking, the centre-back having been sent out on loan to Valencia last season.
Mangala was introduced as a second-half substitute at Old Trafford and Guardiola believes the physical strength of the France international will be an asset, although he was previously close to leaving the club.
"The reason why at half-time I didn't put Mangala on is I wanted to send a message to the players that we wanted to win the game," Guardiola said of Mangala being introduced for Gabriel Jesus - after Nicolas Otamendi put City back in front - with Ilkay Gundogan having been preferred as the replacement for injured captain Kompany.
"I wanted a good build-up against [Romelu] Lukaku and of course Nico [Otamendi] and [Fernandinho] they are stronger in that period.
"After we scored a goal I thought [Zlatan] Ibrahimovic is going to come on so I decided to put Mangala on and take off a striker because I wanted more control in the middle.
"Mangala, when he played for example at Leicester, under the long balls he is a strong player. He is not just going to help us in this moment when Vincent and John are injured but for the rest of the season.
"We were clear with Mangala that we wanted to find a [transfer] solution but when we didn't find it I met him in my office and said 'now you are our player, you will be treated like the other ones'.
"He has a quality that our other central defenders don't have and in some games he is going to help us."