No other Spartak head coach managed to start their work as successfully as Carrera did: Spartak collected 28 of 36 points in first twelve matches under Carrera. On 7 May 2017, Spartak secured their first Russian Premier League title since 2001 under Carrera's leadership.
A very sad day for me personally and for the club as a whole. Carrera probably made some tactical mistakes this season, but he never received any support from the club management.
We sold Quincy Promes, our best player, to Sevilla on August 31, the last day of the transfer window, and did not buy a replacement, and Samuel Gigot, our best defender (and the best CB in the league) torn his ACL. Moreover, our "captain" is too busy with his divorce and does not care about football any longer. Carrera had huge problems with player availability and had to suddenly play many youngsters in the starting eleven.
Moreover, the media and famous club veterans were very critical of Massimo since the summer of 2017, because they don't like that it was a foreigner who led Spartak to the first league title in many years. All in all, no support from the management, no good transfers, no support from the media. The fans backed Carrera up until the end, but it wasn't enough.
We fired Unai Emery in the very same way back in 2012 (the only difference was that he only spent a few months at Spartak), and he went on to win 3 Europa League titles with Sevilla and now coaches Arsenal. I am very sorry for Massimo and sincerely wish him all the best in the future. He never blamed referees, never criticized his players, was always respectful with the media (despite all the criticism). A class act.
Though I am not a Juve fan, I will try to answer your question because I watched every game Spartak played under Carrera and because he said that he wants to return to Juve one day.
This is a very tricky question, primarily because Carrera was never given a chance to build a team he wanted at Spartak. He got 4 players he asked for in the winter of 2016-17, and these transfers helped us improve a lot, but after that the club management sabotaged transfers and even failed to get rid of the players we no longer needed.
Carrera was very flexible tactically in 2016-17 and, to a lesser extent, in 2017-18, when he routinely switched between 3-5-2, 3-4-3, 4-4-2, 4-2-3-1, and 4-3-3, but he was not flexible at all this season, when he decided to go with 4-3-3 and it did not work. His subs sometimes worked great, but sometimes didn't. In the past, he was criticized for poor attacking set pieces, but this season we started scoring tons of goals from set pieces.
Overall, I think very highly of him and think that he has a very bright future, but given that he had to work in a very hostile environment at Spartak, it is hard to tell how he wants to play. IMO Juventus would probably be a bit of a stretch now, and he would be better off going to a mid-table club like Atalanta (but this club has to be well run by the management), and then hopefully in 2-3 years he will be ready for Juve. But he might be ready for Juve even now, who knows...
Carrera reportedly was offered an opportunity to attend one of the Spartak games in the near future to say good bye to the fans. His response at a press-conference today: "I belong to Spartak's past and don't want to interfere with the next manager." As I said, the guy is pure class.