I think Mikkelsen was hired to help Citroen solve some of their problems. Driving for VW last year, and taking part of wrc17 development, made him very desirable choice for that job.
Because of this his main goal with Citroen has nothing to do with how fast he is.
As mentioned alreagy, he had proven how fast he is in R5.
Also, when Citroen asked him to catch Evans at PS in Poland, he almost did so, posting second fastest time, and discarding the efforts of some title contenders.