You don't have to go back too far in this thread (when the false reports came out that United had agreed personal terms with Pepe) to see comments suggesting that this guy couldn't trap a bag of cement, that he needs far too much space to operate effectively (something he won't be afforded in England) and that he's only had one good season in a farmer's League.
Whilst some of these posts were probably made with very little knowledge of the player, it seems strange that we now have posts making out that Arsenal are signing the new Mo Salah.
With regards to Arsenal, it's important to remember they only finished ahead of us due to a horrific start under Mourinho, and even with our horrific finish, we would have finished above them were it not for Mourinho capitulating in preseason whilst throwing the entire squad under the bus (something that some fans somewhat ridiculously want Ole to do now in order to push through new signings).
With regards to their signings, Ceballos would have happily been sold to Arsenal by Madrid. He's made less impression on their team than someone like McTominay here, and people forget that Madrid were so awful last season they would have finished outside of the top 4 in our league pretty comfortably. The only reason he has not been sold outright is due to his own personal request, but Zidane has never fancied him in either of his reigns at the club. (Although from Manchester, I live here in Spain and follow the big clubs pretty closely through media and also through fan channels, so please don't be fooled into thinking that Arsenal are signing the next big thing here. He's not as good as Ramsey by a significant enough distance, so they are certainly weakened by having lost their best midfielder on a free and replaced him with a reserve team player on loan from a Madrid team in need of a rebuild as drastic as the one needed here).
I can't really comment on Tierney as I've never seen him. What I can predict though is the reaction on here is we were chasing him (farmer's League, mid table signing etc).
This is in no way a defence of the movements United have made so far. Whilst some people don't seem to know the definition of a "slow rebuild (which is rebuilding the team by signing 3-4 players every season until after 3 years or so you're challenging for the top prizes at home and in Europe- still need a couple this window which I think will probably just about happen)," let's not be pointing envious fingers at Arsenal, who are still every bit of a shambles as they have been in recent years.