Come on be serious. If you don’t rotate it will haunt you in spring not autumn or winter. Group stage will not depend on rotations but only on tactics.
About being tired in March and April, there are several factors for it:
1. not rotating starting 11
2. playing Copa Del Rey with best players
3. core players are old
We can't do too much about No3.
Imo, we could somewhat survive even without rotations, as long as we would play CDR with squad players.
On the other hand, even if we'll rotate players now and then play extra 6-8 CDR matches with Messi and starters in January and February, we will be dead again in spring.
I would dare to say that Sunday-Wednesday-Sunday matches for 60-90 days in a row in January, February, March are a bigger killer of our team than rotation itself.
https://www.worldfootball.net/teams/fc-barcelona/2018/3/
Last season, a 2nd leg with Roma was our 24th match in 100 days of a New year.
We had:
14 league matches
6 CDR matches
4 CL matches
Without CDR matches, that would have been "only" 18 matches in 100 days.
Which means:
14 weeks of football:
13 weekend La liga matches
1 La liga match midweek
4 CL matches in midweek
6 CDR matches in midweek
With La Liga, CL and CDR, we had:
13 weekend matches
11 midweek matches
So, only 2 weeks "of rest" when we have 7 days between 2 matches.
WITHOUT CDR, we would have:
13 weekend La liga matches
1 midweek La liga match
4 CL midweek matches
= 8 weeks with 7 days of rest
= only 5 weeks with Sunday-Wednesday-Sunday matches
So, again, imo, we could even survive with zero or very low amount of rotation AS LONG AS WE DON'T PLAY Mickey Mouse Del Rey.
If we'll play CDR, then nothing will save us, imo.
Since CDR will come in the worst time of the season.
Our old core of players will get tired in February and we will be walking around a pitch in March, aka in 1/4s of a CL.
So, guys, relax.
You don't need to lose nerves if we don't rotate players in September.
It is irrelevant more or less.
How we will handle CDR in January and February will decide our faith in a CL in March and April.