If the Celtics keep the pick, he may be the BPA at 14. But, if the C's keep the pick, that most likely means no AD trade. And if there is no AD trade that means the Celtics still have Tatum, Brown, Hayward and Smart who all play minutes at the 2 and 3, where Langford would need to play. He would be tremendously redundant.
Given that the difference in talent and quality of prospect at 14 won't be a whole bunch between the BPA and Best Big Available(BPA might equal BBA), I might go with the BBA.
For that reason, I would put Hayes, Hachimura, Fernando, Bol or Doumbouya in front of Langford and pick from one of them, if available at 14.