ALLEN PARK, Mich. (AP) - Chris Houston sat out because of a migraine, and Louis Delmas is still nursing a balky knee. The Detroit Lions hope their secondary will be much improved this season - but first, they need their top players healthy and on the field together. Houston and Delmas both missed Monday's practice, and although there were plenty of other defensive backs eager to take their place, the Lions are well aware that it's important for cornerbacks and safeties to develop chemistry before the season starts. That's not easy when key players are sidelined. Detroit re-signed Houston, a cornerback, and Delmas, a safety, in the offseason. The Lions also signed free agent safety Glover Quin and drafted cornerback Darius Slay in the second round.