I could be wrong about this, but Rodri would get away with not needing surgery if it was just strained or over-stretched, right? I seem to remember Kevin missing the first chunk of the 18/19 season because of an ACL injury but he just needed to rest it instead of going in for surgery, which meant he came back about three months earlier than expected. Like I said, I could be wrong about that.Your last sentence is correct.
ACL injury at elite level means surgery. Even in an incomplete injury the ligament is stretched and is extremely unlikely to recover normal function. Any further injury can then result in meniscus damage so not worth taking the risk.
What I do find strange is that Leroy Sané tore his ACL in August 2019 and was back in full training by January 2020, but then never played another game that season - and TransferMarkt has his return date down as March. I can't remember how those circumstances played out - maybe Pep just didn't want Leroy too involved because his head was out the door by then - but if Rodri can be back playing even by January or February then we'll be incredibly lucky.