KANSAS CITY, Mo. — Chiefs coach Andy Reid had expressed optimism that safety Tyrann Mathieu would be back on the practice field Thursday. A day later, Reid’s words proved prophetic. Mathieu, who suffered a concussion in last Sunday’s playoff win against the Buffalo Bills, was able to practice on Thursday alongside his teammates as the Chiefs prepared for Sunday’s AFC Championship Game against the Cincinnati Bengals at Arrowhead Stadium. He was observed going through stretching/conditioning and individual position drills during the brief period of the practice that was open to the media, and hi…