The ace of the rotation, Danny Duffy, is currently still with the team. As long as he does not get traded before the year begins, he will be the teams number one starter. The number two and three starters, in no particular order, will be Ian Kennedy and Jason Hammel. That is a solid front end of a rotation, regardless of how last year went.
When looking at recent history, it is hard to imagine Kennedy and Hammel replicating 2017.
Over the last two years, Duffy has a 3.64 ERA as a starter, going 21-13 over 307.0 innings and 50 games. Kennedy had an abnormal 2017 season. From 2014-2016 Kennedy posted a 3.84 ERA, going 33-39 over 565.0 innings and 96 games.
Hammel also had an abnormal 2017 season. From 2014-2016 Hammel posted a 3.68 ERA, going 35-28 over 513.2 innings and 92 games.
When looking at recent history, it is hard to imagine Kennedy and Hammel replicating 2017. Both had their reasons for why they struggled. Injury, adjustment to a new team, not having the teams regular catcher for a significant chunk of the year are all legitimate reasons. Therefore, it is a safe bet that they will fall back to what recent history shows rather than repeat what 2017 showed.