While the Rays' top trio of James Shields, Scott Kazmir and Matt Garza were a virtual lock, it was thought that Jackson and Andy Sonnanstine were in contention for the fourth and final spot, with the other heading to the bullpen.
A visibly disappointed Jackson felt the same way, calling Tuesday's decision a "surprise."
"I thought maybe I would be in the 'pen," he said, "but not being on the roster, that hadn't crossed my mind."
Jackson said there was no talk of putting him in the bullpen, and when he received the news on Tuesday, he was told only to keep throwing in case the Rays need to activate him.
The club informed Jackson of "circumstances that have to happen for me to get back for the first round," he said. "So [they told me to] just try to stay sharp as I can. It's always hard when you are not playing to stay sharp."
Rays manager Joe Maddon did not initially comment on the state of the bullpen, choosing to unveil only the club's starting rotation. However, upon hearing of Jackson's comments, Maddon did not refute them, Instead the skipper said the plan to leave Jackson off the roster is not definite.
"It's as of right now," Maddon said, noting that things could still change since team rosters do not have to be finalized until 10 a.m. ET on Thursday.
Jackson had a career-high 14 wins this season, and posted a 4.42 ERA. However, his 77 walks were more of a concern, as Maddon cited Sonnastine's consistency and "strike-throwing ability" as key factors in the decision.
Sonnanstine, who is slated to go in Game 4, if necessary, on Monday, said he was "excited at the opportunity" to start in the postseason.
"I did my best to make every single start this year," he said, "and felt like I helped the team out a lot."
If the Rays opt to go with 11 pitchers, and assuming Jackson is off the roster, there are several scenarios that could play out.
The Rays would have seven relievers, which could include both rookie David Price and veteran Troy Percival. While Maddon admitted he had several talks with players on Tuesday to give them a heads-up of the forthcoming decisions, no official roster announcements are expected until Wednesday at the earliest.
Brittany Ghiroli is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.