According to The Mirror in the United Kingdom, all signs still point to John Cena taking on The Undertaker at WrestleMania 34.
Sports Illustrated’s Justin Barrasso reported last week that Cena would face Rey Mysterio. However, Mysterio reportedly sustained a biceps injury during a tag team match last Friday in a Northeast Wrestling show in Waterbury, Connecticut. It is unknown what the status of Mysterio is currently and whether he will even come to terms with WWE. 
Getting back to the match with Taker, Cena is expected to ramp up promo efforts for the match after Fastlane. WWE is using Cena to add interest to what has been rather stale storylines on the Blue Brand as of late. Cena certainly adds to the spectacle of a pay-per-view show which has yet to generate a lot of buzz so far. After Fastlane, Cena will reportedly crank up the volume on WrestleMania. 

Obviously, the WWE Universe will not be satisfied until the match is for certain, but many insiders believe The Undertaker will make a comeback.