MINNEAPOLIS -- Other than veteran right-hander Ervin Santana, the Twins' rotation struggled in 2016, combining to post a Major League-worst 5.39 ERA.
The Twins used 11 starters, and only Santana had an ERA of less than 5.00. And no starter reached 10 wins. Improving the pitching staff has been a priority this offseason, but outside of adding defensively-minded catcher Jason Castro and selecting Justin Haley in the Rule 5 Draft, Minnesota hasn't done much to add to its existing rotation.
The Twins have several internal rotation candidates, though. Here's a look at the starting pitchers, outside of Santana, currently on Minnesota's 40-man roster:
RHP Kyle Gibson
It's telling that Gibson was the Twins' second-best starter despite posting a 5.07 ERA in 25 starts. The sinkerballer couldn't build on his strong 2015 season, during which he had a 3.84 ERA, and dealt with back issues that limited his effectiveness. This will be a big season for the 29-year-old, who has struggled to put hitters away with his secondary pitches.

RHP Phil Hughes 
After a down year in 2015, Hughes came into Spring Training in better shape in '16, but he dealt with thoracic outlet syndrome that led to season-ending surgery in July. Hughes is on track to be healthy for Spring Training, and Minnesota is hopeful he'll bounce back from the injury.
LHP Hector Santiago
Santiago was acquired from the Angels at the non-waiver Trade Deadline for Ricky Nolasco, but he was inconsistent after joining the Twins, posting a 5.58 ERA in 11 starts. He struggles with walks and home runs, but the lefty has a career 3.84 ERA, so he can be effective. Santiago gives Minnesota a veteran southpaw in their rotation, but he needs to improve on his uneven 2016.

RHP Jose Berrios
Berrios was promoted to the Majors in 2016 as a highly-touted prospect, but he had a forgettable rookie season, with an 8.02 ERA in 14 starts. Berrios had trouble with his command, especially with his fastball, which caused him to fall behind too many hitters. The Twins still have high hopes for the 22-year-old, but they might send him to Triple-A Rochester for more seasoning.
RHP Trevor May
May has been a strikeout machine in the bullpen -- notching 60 in 42 2/3 innings last year -- but he's had trouble staying healthy since moving to a relief role. May has dealt with back injuries over the past two seasons, and given his history as a starter, he'll get a chance to come into Spring Training competing for a spot in the rotation. May has the size and stuff to be a starter, but this might be his final chance to prove it with Minnesota'.

RHP Tyler Duffey
Duffey followed his strong rookie year with a rough season that saw him post a 6.43 ERA in 26 outings. Duffey tried to incorporate his changeup more, but he is primarily a fastball-curveball pitcher. Given his lack of a third pitch, it could make sense for Duffey to move to the bullpen, but he'll at least get the chance to make the rotation in Spring Training.
LHP Adalberto Mejia
The Twins acquired Mejia from the Giants for Eduardo Nunez at the non-waiver Trade Deadline, and the 6-foot-3 lefty is considered close to being Major League-ready. He had a combined 3.00 ERA in Double-A and Triple-A last year, with 126 strikeouts in 132 innings. He's likely to open the year in Triple-A, but Mejia is expected to be in the Majors at some point this season.

RHP Justin Haley
Haley, who was selected in the Rule 5 Draft from the Red Sox, will have a chance to compete for a spot in the rotation, but he's more likely to be used as a long reliever. Haley had a 3.01 ERA across Double-A and Triple-A last year, striking out 126 in 146 2/3 innings.