At this point, the Celtics might not see Josh Jackson unless they draft him. A team source confirmed an ESPN report that the Celtics were unable to arrange a workout with the Kansas forward yesterday. The source added, “Not yet,” when asked if the Celtics still hoped to schedule a session with Jackson before tomorrow night’s draft. According to ESPN, Jackson wants assurances before agreeing to a workout, including a promise the Celtics will hold onto the third pick, and that they will take him. The Celts, though, aren’t that far along in their evaluation process, or in Danny Ainge’s exploration of trade possibilities, including yesterday’s NBA sun-blotter that the Knicks are open to trading Kristaps Porzingis. Ainge, as the personnel chief with the most assets to offer, including the third pick in the draft, was sure to be right at the edge of the ring, throwing in his hat. But in the absence of a draft-week deal like Ainge orchestrated for Ray Allen in 2007, the Celtics still have a choice to make with the third pick. As of yesterday, the front office was undecided and still is weighing the merits of Jackson and Duke’s Jayson Tatum. As Ainge said earlier this week, the team has both players ranked as high as Markelle Fultz and Lonzo Ball at the top of the draft board. Though Florida State’s Jonathan Isaac is said to be on the periphery of Celtics’ thinking, thanks to the 6-foot-11 forward’s defensive versatility and developing jumper, the team is thinking the hardest about the following two players: Jayson Tatum — The 6-foot-8 Duke forward won over the scouting staff in several ways, including one of the most polished scoring capabilities and an off-court personality that has not just impressed the Celtics, but every team within interviewing distance.