I can't believe that's all there is to it - there must be something that went wrong between Dele and Jose in the first half of the season. I say that because Dele was always starting under Jose last season, there were no real rumours of him leaving the summer, and he started the first game of this season. Then for some reason Jose subbed him at HT, and hasn't started him again in the PL since.
Personally I suspect that Dele didn't react well to being subbed at HT on the opening day, and/or to his subsequent limited gametime (in attitude and/or performance), and by January Mourinho was tinkled off that Dele hadn't reacted in a positive way. (At that point I can believe that he wanted to sell him, including to generate funds for a centre back).
Let's hope the window closing, and Dele's recent gametime and good EL performances, are the start of him playing more first team football and getting back towards his best.