Danny Duffy will only miss a couple of days with this tightness, per Jeffrey Flanagan. And that was as of two days ago. It should be no big deal, as long as it does not linger. The Kansas City Royals could use the southpaw healthy and at the top of their rotation in 2019.
Tightness like this can come with age. Pitching is a very hard thing to do and it does not get easier on the arm the older pitchers get. In fact, Duffy compared his shoulder issues to that of former teammate James Shields.