I'm not sure what "used to" means. For what it's worth, this is what a link to a player's milb.com page looks like (and has since at least the beginning of March):
https://www.milb.com/player/david-dahl-621311
This is what the link on the box score page looks like:
http://m.mlb.com/player/621311/david-dahl-621311?_gl=1*1xz7307*_gcl_au*MTA0NzcwNDM0MS4xNzE2NjA5NTky
The section "david-dahl-621311" is correct; the rest of it... well, it's a link to a mobile mlb.com site (thus the "m.mlb.com" part); on my desktop browser (Chrome), when I try to access http://m.mlb.com, it takes me to the non-moble mlb.com home page. Same thing happens on my phone (using Safari) - it takes me to a mobile presentation of mlb.com, but doesn't show the site address as m.mlb.com.
Given my overall opinion about the quality of the people maintaining the mlb.com web sites, I would expect this is a site programming error.