There are some murmurs that Brian Callahan could be headed to Tennessee to fill the OC position on Mike Vrabel’s staff. This is one reason why Ryan Day may have remained at OSU instead of joining the Titans. We should find out fairly quickly. 

http://profootballtalk.nbcsports.com/2018/01/23/report-lions-parting-ways-with-quarterbacks-coach-brian-callahan/