"The Browns talked to him and asked him to play this week; he declined," MMQB's Peter King wrote in his Monday column.
"There are going to be other guys that are not participating either, so I won't hold it against him," Jackson told cleveland.com. "I don't think that's the right thing to do. He's still a draftable player, there's still a process we have to go through to evaluate him and see if he can fit here with the Cleveland Browns."
The Senior Bowl gathers more than 100 prospects each year for a week of practices, followed by the all-star game, in Mobile, Ala. Evaluators from all 32 teams attend to watch and meet with prospects that participate in the event.
"Obviously we wish he were here," Jackson said. "It would've been a great chance to have an opportunity to evaluate him doing the things we do in meetings and on the practice field. But that's not going to happen, so we'll have to find a different way of evaluating him."
Watson completed a grueling a 15-game season on Jan. 9, when he led Clemson to a national title victory over Alabama.
NFL.com analyst Daniel Jeremiah said after Watson's decision was revealed that he didn't believe anyone in the NFL would have a problem with Watson's choice. However, he also said Watson could've helped himself by competing -- it would've been a unique setting for Watson to compete in, the likes of which won't be replicated at the combine or his pro day. Jeremiah has Watson going to the Texans at No. 25 in his first mock draft of 2017.