2nd round pick Robert Darby should make an impact in nickel situations as a rookie as he learns behind Stephon Gilmore, Leodis McKelvin, and Corey Graham. You can never have too many corners and Darby fits in the Rex Ryan mold as he values lockdown corners tremendously in his defensive scheme. McKelvin has a high price tag and could be a cap casualty after this season. Graham can be moved to safety if the corner depth chart gets to crowded. The knock on Darby is his ability to close a play, eerily similar to the knock on McKelvin.
The other two draft picks that can have an immediate impact are 3rd round pick John Miller and sixth round pick Nick O'Leary. Miller has a chance to step right in and start as a rookie at left guard. He is a mauler who fits the Greg Roman ground and pound mold perfectly. Roman's offensive scheme also uses lots of tight end sets and the team has said they may carry four of them. O'Leary is a good complement to starter Charles Clay and can lineup on the line while Clay plays the H-back.
5th round pick Karlos Williams was a polarizing pick as he comes with a lot of off the field baggage and is at a position of strength. It appears like he will battle Bryce Brown for a roster spot. Since Ryan doesn't seem enamored at all with Brown, and his regime drafted Williams, look for the rookie to play but be inactive most weeks. 6th round pick Tony Steward adds needed depth to inside linebacker but even with that going for him, he will have a tough time making the team. 7th round pick Dezmin Lewis is an intriguing pick who many projected to go a couple of rounds earlier. He is also at a very crowded wide receiver position which means the practice squad is likely his best bet. He is the tallest receiver on the roster which may make them tempted to keep him around as a red zone threat.
The Bills added 13 undrafted free agents who will add more depth for training camp but as the team continues to improve their will be less contribution needed or expected from rookies. I do expect Williams to have the biggest immediate impact as the team does have a hole at the starting left guard spot, if he starts the whole season it will make the draft a success on that alone.