I believe that is being flown with a 747 now...and I don't think the 787s are planned to pick up any ORD flights in the near term.

With United's seats in economy, 747 would probably be a more comfortable ride anyway over the cramped, slim, narrow 787.
In business, the Upper Deck of 747 is pretty sweet.

Any particular reason you are wanting a 787? Seat comfort would go down...