Except Johnson had actually proved in the past that he could be an excellent bowler, apart from 2008 when batsmen couldn't pick him Mendis hasn't ever really looked like a threat at test level. You tell me all the time he needs to be given proper chances and needs patience and time ect, but we can't argue because I don't think he has the attributes a test spinner needs while you obviously do.
The other issue with the Mendis selection was that he really didn't do anything to get selected, granted, he's bowled quite well this time around, but the chance to bowl to Bangladeshi batsmen on a spinning wicket should've gone to someone performing on the domestic circut like Tharindu Kaushal or Suraj Randiv.
And tbf, Mendis is being treated weirdly by the selectors, getting one test here, another here (although in that last Bangladesh one in Galle you mentioned he actually bowled trash - went at over 5 an over) so I too would like a bit of clarity from the selectors, either give him a proper run or get him away from the test set-up, my preference would obviously be the latter.