Slav, I think it begins next offseason. Your 2019 will be a mostly intact 2018 club minus Kimbrel. You didn't need Eovaldi to win 108 games. Losing Kimbrel and just the relative impossibility of winning 108 games in back to back seasons will lead to some sort of regression, but the sox will be a mid 90s to 100 win team again next year barring an ultimate collapse. But there is literally no way to weather the 2019-2020 offseason without significant losses on the big league roster with more coming. And it isn't like the sox can plug and play another Beni or ERod into the spots of guys moving on.
I also hear about retaining Eovaldi a lot and while his post season heroics were insane, the guy's regular season track record for 2018 wasn't that great. He averaged less than 5IP for the sox. His ERA was good, but his starts were marred by high pitch counts. Overall, he threw 109IP in 21 starts, which comes out to a tick above 5IP per start. Let's say the plan is to re-sign Eovaldi and let Porcello walk after 2019. Porcello has averaged 197.3IP over 4 seasons in Boston with an average of 32 starts per season. His IP/start average is 6.2IP (actually 6.2, ie removal with one out in the 7th). Eovaldi has averaged 6IP per start once in his career and has hit 30 starts once in his career. With the Yankees he was at 5.47IP per appearance. In 2018 overall, he averaged 5.1IP per start. By replacing Porcello with Eovaldi, you are likely to save some money on a per annum contract, but you have to assume that the pen will need to work an extra inning per start. That extra inning is valuable, clearly. Also the extra starts are valuable. Eovaldi made 27 starts and then 21 starts with the Yanks. Those are his second and fourth highest start totals respectively. Replacing Porcello with him will lead to more 6th starter starts and one inning more of relief in games he can toe the rubber.
The other thing the sox will have to deal with is what to do with Chris Sale? If Sale comes out firing in ST and has his usual season of 200IP, dominant starts and then fade late, do you sign him to an ace level deal? Clearly, if he doesn't go out and throw tons of innings, he isn't gonna be re-signed, but the question marks with Sale have mostly been muted by the unexpected performances of the other guys in the rotation and haven't been entirely addressed. If you re-sign Sale, he will eat a ton of your budget. If you don't, it will leave a big hole at the top of the rotation