We need hundreds of millions of investment in this team to get back on top. Far too much being asked of Messi.
We've lost our only really saleable asset though (given Messi's worth nothing now with his contract).
I think the thing we need more than money is a good scouting team. The talent is there, but we seem no not recognise it. Also our coaching stuff must give young players more chances at least in the league. Our office has some kind of tunnel vision when it comes to buying players. They wanted Coutinho and they failed. After the first bid was rejected, we should have turned to other options, but there weren't any because the board didn't do their job properly (Ironically, Liverpool did the same with Van Dijk and as the result we are stuck with ageing Iniesta and bad Rakitić and they are stuck with terrible Lovren). I hope we sign Goretzka in January, because Schalke will do anything to sell him. He said he wants Bayern or us, so we should have some advantage, because Schalke will be reluctant to sell to Bayern. If the reports are true, we can sign him for 10-15 million which is a steal for a young promising midfielder (unless Bayern bumps up his price with biding). Also if we scout a promising young player (like we did with Semedo this summer), we should go for him instead of 25+ old expensive players.
What players do we need?
Goalkeepers: We are good here for quite some time.
Defence: We need to buy/promote a CB (or recall Marlon) and buy a backup true RB (or promote Palencia). We also need to sell Vermaelen.
Midfield: We need a playmaker, a controller and an all-round midfielder. If we can find a player that ticks two of those, better for us. My favourites: playmaker: Lemar/Alena/Coutinho, controller: Seri, all-round midfielder: Goretzka. If we can get Goretzka for 15m in January and Lemar for 90m in the summer, our midfield would be instantly better. Also we need to lose the deadweights (Gomes, Rafinha, Turan) and sell players who are likely to drop in their value (Rakitić).
Attack: We need a striker who will replace Suarez. Preferrably one that can also play as a LW. Anthony Martial and Marcus Rashford are names that spring to my mind. If we can ship any of the deadweights or Rakitić or even Deulofeu as a part of the deal, we should do it. Also sell Paco.
But the season is still long and there can be young players who will show their potential during the following months.
I also believe that if Messi leaves in the summer, Bartomeu will need to run for his life out of Barcelona.