Justin Smith’s full value to the 49ers was best displayed after his triceps tendon was half torn. The Niners would have identified their defensive line as an area they needed to address in the draft, but their performance after Smith’s mid-December injury surely heightened the sense of urgency. What would life be like with an absent or diminished Smith, who will turn 34 in September and is entering the last year of his contract? The 49ers got a harrowing peek into the future when the All-Pro defensive tackle was injured early in the third quarter of a 41-34 win at New England, missed the final two regular-season games and returned for three postseason games sporting a bulky brace on his left arm. Before the Super Bowl, Smith said his triceps was at least 50 percent torn. In their first 13 1/2 games, the 49ers’ defense allowed an average of about three points per quarter. After Smith was injured, they allowed about seven. Similarly, opposing quarterbacks had a 75.3 rating before Smith’s injury. That figure ballooned to 100.6 in the final 22 quarters after he was sidelined. Finally, outside linebacker Aldon Smith didn’t have a sack in the final six games after posting 19 1/2 in the first 13 regular-season games while working in tandem with Smith. Beyond the need to find Smith’s heir apparent, there are other reasons to address the defensive line in the draft. Nose guards Isaac Sopoaga and his backup, Ricky Jean Francois, are pending free agents. In addition, Jean Francois is the only backup lineman to log an NFL start. The rest of the reserves – Will Tukuafu, Demarcus Dobbs, Ian Williams and Tony Jerod-Eddie – are undrafted free agents. The lack of depth at the position explains the heavy workload of Smith and defensive tackle Ray McDonald in recent years. Including playoff games, McDonald ranks third in snaps played by defensive linemen since 2011 and Smith is fourth, according to ESPN. The 49ers haven’t selected a true defensive lineman in the first six rounds since drafting noted bust Kentwan Balmer in the first round in 2008. That streak will end this season, and it’s a good year for the 49ers to shop: The draft is loaded with defensive linemen, and San Francisco has four of the first 93 picks over the first three rounds.