The sooner the Lions and quarterback Matthew Stafford agree on a long-term contract extension, the better.
Stafford has two years remaining on his original contract, but it does limit what the Lions can do in free agency in coming years. A long-term contract would spread out the salary cap hit. Stafford's original contract was typically back-loaded and the Lions are looking at huge cap hits this season and next approaching $20 million each year,. It didn't hinder the Lions this off season in signing free agents because they essentially kicked the salary cap hit for defensive tackle Ndamukong Suh down the road. In the future, it will be different. Suh,and Stafford would both have huge hits next season.

Stafford: No reason  to believe contract will not get done

Also, there is the necessity to avoid a distraction. It hasn't gotten to that stage yet, but if the Lions start this season, and Stafford is not signed long term, it will gradually become the top issue surrounding the organization.
Stafford is the easily the best quarterback the Lions have had since the 1950s. Last season, the Lions were 4-12, but it's difficult to place the blame on Stafford. It took the Lions 50 years to find a QB of his caliber and attempting to replace him would be risky. It's not just the comeback victories he has led (including three even last season) or the passing numbers (5,000 yards or close to it the last two years), but also Stafford has embraced playing in Detroit since Day 1. He likes it here, wants to be part of a Lions' uprising on the field, and understands his position perfectly well as a leader. He's an NFL player, meaning any guaranteed money he receives from a contract extension would be wise for him before he risks injury in the coming season.
The Lions' problems have been with talent evaluation and poor planning putting together a roster on the field. They have actually done well with contract negotiations. The one talent evaluation they did get right was drafting Stafford first overall in 2009 (anybody still want tackle Jason Smith or linebacker Aaron Curry or QB Mark Sanchez). They can't afford to let Stafford fall by the wayside, especially given the importance of the position. And Stafford wants to stay. I believe, probably sometime between the NFL Draft and the start of training camp, this will get done.