source: Wrestling Observer Newsletter

At the moment, Undertaker is slated to return in late July. The storyline will lead to a SummerSlam showdown with Edge.

Taker did indeed take some time off to heal some nagging injuries, but there was also a feeling that his program with Edge needed to be put on the shelf, at least for a little while. They want to continue it, but it was going to run out of steam without a major angle.

The creative team wasn't told about Undertaker's leave of absence until a week ahead of time.

The nagging injuries don't surprise me. He used to work a part-time schedule because of them, then he worked full-time for quite a bit and there's no way they had magically disappeared.