There are two main concerns over Jacquet. The first, naturally, is how would he scale up to a league and team with much more scrutiny on them, particularly if he is being heralded as Chelsea's defensive saviour in the long term. Just because he's made Ligue 1 his playground doesn't necessarily mean he will succeed in the Premier League, and the more hype he receives, the bigger the target on his back will be.
The second is to do with his natural game. Jacquet's aggression, for the most part, is within reason and he can get away with his touch-tight tendencies. But when it goes wrong, it can spectacularly backfire. It's not uncommon for him to get spun or give up a cheap foul, and it's a skill so particularly hard to master that this might always be a weakness.
Earlier this season, Jacquet picked up his first career red card during Rennes' 5-0 defeat away at PSG. Already on a yellow card, he swiped the ankles of Goncalo Ramos, who had nipped in to cut out a loose pass at the back. This was more of a concentration issue, but in his first game back for Rennes, Jacquet picked up another booking to trigger an automatic suspension.
Questions over his discipline remain valid, and heading to a team as hot-headed as Chelsea - bottom of the Premier League's fair-play table - may not aid his development in this regard.