The Chiefs' pass defense may look good on paper, but their secondary has had many issues, especially in man coverage. Chiefs' number one corner, L'Jarius Sneed has played well this year but gives up a lot of yardages, especially in terms of penalties as he's the most penalized defensive player in the NFL, recording 15 penalties.
Most of those penalties are defensive holdings or pass interferences, to which he can get very handsy when playing man-coverage on somebody. Stefon Diggs can surely draw lots of penalties when playing against Sneed and maybe beat him on some deep routes, as he gets beat a decent amount of times. So it's good to target Diggs or even Davis in one-on-one situations against Sneed.
There may be times when Chiefs star cornerback Trent McDuffie will be covering Diggs. McDuffie is having a Pro Bowl-caliber year so far, and Diggs will have to step up every time he's getting covered by him (which is usually in the slot). Although, everyone expects Diggs to, as he's one of the best in the league at his position.