You're not going to replace Coutinho's magic.
The thing is I liked the shape and balance of your team last night. There were players that fit in the right positions, good combinations, you just lack the quality to be a team that can produce consistently enough to properly challenge.
Gomez was the worst player on the pitch for me. He saw so much of the ball going forward because Salah plays very narrow and he created nothing for the forwards all game and was easily Swansea's best route to a turnover and quick counter. Robinson wasn't as bad but not much better.
I like the power, athleticism and pace you had in the midfield three of Can, Chamberlain and Wijnaldum - it's the quality that's the problem. Chamberlain was terrible, other than a few good set-pieces, Wijnaldum does little in either direction and Can was trying a bit too hard, although I thought he had a really good game actually.
I know you'll probably disagree but Mane is a disaster for you. He's so erratic in everything he does. Makes poor decisions over and over again. He might put up good numbers but he's not a very good player. He'll hold you back if you want to try and challenge City.
Salah was class in nearly everything he did, but the guy having no right foot limits him now that everyone's terrified of his left foot. His combination play was wasted by his team mates last night.
Firmino's miss at the end was awful, and he had a poor game. When his link play isn't perfectly on point, you really start to see his lack of power and speed. When you have 65%+ of the ball he spends too much time trying to battle with big centrehalfs and he can't cut it. Very similar to the struggles Lacazette has had for us.
If you get Clyne back, stick Keita in for one of Chamberlain or Wijnaldum, spend what money you have left on replacing Mane with a much better player yoid be a much team for it.
But you're going to lose Can - which is a big problem, and putting that aside, to really challenge next seaaon, you would still desperately need a goalkeeper (that goes without saying!), a better starting left back, ideally a better partner for van Dijk than Matip, and a very good alternative to Firmino, perhaps even a replacement becauae he just doesn't have enough goals in him across a season.
Surprised you're not trying for Aubameyang too if the £53m price tag is to be believed. He'd take you up 3 levels alone.