Source: 49ers have done extensive homework on DL Vernon Butler

Of the 63 defensive linemen at the NFL combine, Vernon Butler was the fourth-heaviest (323 pounds) and his arm length (35 inches) and hand size (10 3/4) tied for fourth among his peers. At Oregon, Kelly recruited 49ers defensive tackle Arik Armstead and defensive tackle DeForest Buckner, a projected top-five pick in this year's draft. Butler, who had 29.5 tackles for loss and five sacks in his two seasons as a starter, is know for his versatility and could play any up-front spot in the 49ers' 3-4 defense. The 49ers could be without Ian Williams to open the regular season after the nose tackle underwent ankle surgery this offseason. Defensive tackle Glenn Dorsey, who played nose tackle in 2013, tore his ACL on Nov. 22 and his season-opening status in also in question. The 49ers' other defensive linemen include Armstead, Quinton Dial, Tony Jerod-Eddie and nose tackle Mike Purcell.