Antonio Conte's side have recovered brilliantly after their opening-day defeat to Burnley - and they were only side to beat City home and away last season.
Mendy seemed in strong spirit despite the injury and tweeted he was joining Loan FC.
"Our first impression is that this will be a bit long, longer than what we imagined, which is a shame", said Guardiola after the victory of the Citizens against Donetsk in the champions League (2-0).
Earlier on Thursday night, Aguero posted a picture of himself on Instagram with the Colombian singer/songwriter Maluma.
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Mendy has made five appearances in all competitions for City in 2017-18, providing one assist.
However, the 23-year-old Frenchman suffered a nasty-looking knee injury during City's 5-0 win over Crystal Palace on the weekend and fans feared the worst as Mendy was immediately subbed off and soon seen trudging around in crutches.
The prospect of losing Mendy for the rest of the season severely weakens Guardiola's strategy, as the France worldwide was a key part of his summer squad rebuilding.
"We have alternatives in our squad - Danilo, Fabian Delph, Fernandinho, can play there, so we will see as we go along".