The Opening Day starter and rotation
SP Marco Gonzales
SP Justus Sheffield
SP Yusei Kikuchi
SP Chris Flexen
SP Justin Dunn
SP Julio Teheran
With Chris Flexen signed the Mariners six-man rotation looks like it is coming to shape, and at the top facing the San Francisco Giants and (most likely) opening day starter Kevin Gausman, is Marco Gonzales.
It seems as if Seattle will continue to have an offseason that does not include a big-time signing, so I expect that what they have right now is it, except they bring in Julio Teheran in a depth role. I could see the Mariners signing Teheran to a minor league deal or a prove-it deal similar to Taijuan Walker, and then he beats out Margevicius for the last rotation spot.
Sheffield, Kikuchi, and Flexen are locks to make the rotation and then there is a group of Justin Dunn, Nick Margevicius, Julio Teheran, and Ljay Newsome. Dunn and Teheran will take the last two spots but Newsome and Margevicius will be the first ones called up when someone inevitably gets injured.
The Mariners used a six-man rotation in 2020 and Scott Servais recently said that they will use the same system in 2021. He also said that he would not be surprised if the Mariners signed another pitcher to either a depth role or a rotation role, hence the Teheran signing.