In short, here is what we are seeing:
1. Bullpen is being asked to pitch too much and it's taxing them. That is the fault of both the starting rotation and the fact that there hasn't been a long reliever out there until now.
2. Guys that shouldn't be starting are getting too much playing time because of injuries (i.e. Grossman, Adrianza, LaMarre). These are guys that play better when used less. You play them this much, you see why they aren't starters.
3. Underperformances by a number of key players. To name a few: Dozier, Castro, Rosario, Morrison, the whole pitching staff. These guys can't be this bad all year.
1. Twins relievers have the 8th fewest innings pitched in MLB so far this season. Mostly a function of having the fewest games played, but even on a per-game basis, we are within 1 out of MLB average relief innings per game (MLB average is 3.2 IP, Twins at 4 IP). I'd guess we've used more total relievers than average too, so the workload has been spread out a bit.
Here are the IP ranks of individual Twins relievers:
2. Grossman started 75% of the time for the Twins in 2016 in 2017. He has only started 56% of the games in 2018 so far. I don't think over-exposure is the cause of his 2018 struggles. Adrianza has indeed started more in 2018, but he's only 2 starts ahead of his 2017 rate so far. LaMarre has only started 6 games all year. It's not like previous Twins teams haven't seen marginal guys get similar playing time (don't forget, the 2017 team had Danny Santana to open the season, later had mediocre performances from rookies Garver and Granite, and even had Adrianza start 9 games in the outfield).
3. Castro and Morrison are definitely under-performing, but Dozier and Rosario are within 10% of their career OPS+ figures. On the pitching side, Lynn and Rogers are really the only regulars who are under-performing expectations.