I don't think Pep Guardiola will be as worried as you say. Manchester City have players who can still be relied on like Foden if Bernardo Silva leaves during the transfer market. Pep Guardiola also has good defenders where it is possible to replace Walker who other teams are interested in, but of course it is important to try to keep those two players as much as you can.
Even though Manchester City ended up having to part ways with their 4 important players during the transfer market, so I don't think they will lose a lot of quality which can make their title chances lower. Pep Guardiola definitely has a good plan in mind for his team, he will also anticipate other plans should Walker and Silva leave.
Yes, there are a number of things that you should pay attention to because a big team will lose balance if many of their core players leave in this summer's transfer window. Manchester city just made their history last season by winning the treble winner. So there are several steps that pep must anticipate to balance their strengths in the next season, where he must be able to persuade star players not to leave in this transfer market.
Indeed pep guardiola has some stock players but if some influential players in the depth of their squad leave in this transfer window then it is very likely that manchester city will also lose its best form next season. Therefore I think pep guardiola has to hold Walker and Silva to stay with them next season.