We're behind in current footballing terms, not in club reputation/size/viewership/history/etc. Messi going there now means they'll get a feck ton of fans who flock to see him, you'll have loads of kids growing up wanting to be like Messi, loads of new sponsorship opportunities, they're claim to having lured the best player of all time to play for them, and now the best possible chance at finally winning the Champions League. All of this not only brings them up, but also drops us down as it'll be like officially we are no longer the biggest club in Manchester, loads of those fans will leave United for City, we'll lose sponsorship opportunities as they'd rather pick City ahead of us now due to viewership/appeal and whatever else.
If there's anything that can create a big shift in the overall club size/reputation, its this.