EDEN PRAIRIE, Minn. Vikings coach Leslie Frazier remembers the explosiveness receiver Jerome Simpson displayed early this season, including a highlight leap over a defender in the first preseason game that was a salivating glimpse at the athleticism everyone hoped Simpson would bring to Minnesota.But since Simpson's mysterious legback injury he suffered more than a month ago, the coach doesn't see that same burst from Simpson. Frazier believes the fifth-year receiver still offers help to the Vikings' struggling passing game, but Frazier doesn't think this is the completely healthy Simpson he saw show off unique speed and leaping ability earlier this year."He's not where he was prior to that injury," Frazier said. "He's not. You look at the tape and you know that. But he's still able to give us some things, but he's not where he was before the injury, for sure."Simpson, signed during the offseason on a one-year contract, was a bright spot in minicamp and the preseason, clicking immediately with quarterback Christian Ponder and looking exactly like the deep threat, big-play receiver Minnesota was looking for.He had to sit out the first three games because of a suspension but again demonstrated his athleticism in his first game back against the Detroit Lions when he caught four passes for 50 yards and added another 57 yards in pass interference penalties. One of those catches against Detroit, which comes to Minnesota this week for the second game between the two NFC North opponents, was another spectacular leaping catch over a defender."That's like a trademark of his," Frazier said. "(The injury has) taken a little bit of his explosiveness away. When that will come back all together, doctors can't tell me. But what he has, he's still giving us something. But it's not the same Jerome prior to the injury."Simpson, who has been hesitant to discuss his physical condition, agrees with Frazier."It's holding me back just a little bit, but I think I'm getting better and better every week," Simpson said.Since the first game against the Lions, Simpson has four catches. For the season, he has eight catches for 109 yards. The lack of explosiveness could be contributing to Simpson's inability to gain much separation from defenders.A healthy Simpson would likely make a big difference in the team's struggling passing game. He's certainly still affected by the injury, though Simpson and Frazier have both said it's not an issue of pain."That's part of it, being able to push off on certain cuts," Frazier said. "It does affect him being able to just sprint and open up the way he was doing before the injury. So, it's taken a little bit away from his game. He's still able to give us some things, but it has definitely affected him."Simpson is simply trying to contribute."I'm just playing the game and just trying to be the best player I can be," Simpson said. "That's about it."Ponder, who had that instant chemistry with his new receiver, offered another reason for Simpson's limited impact."I think the biggest thing people don't realize about Jerome is that he spent three or four years in a specific offense," Ponder said. "He's still growing. He's still learning this offense. We're seeing he's getting better every week, and we're looking for him. He had a great game against Detroit last time, so we want him in that same position this week."Carlson returns to practice: Tight end John Carlson, another free-agent signee who was expected to help the passing attack, returned to practice Wednesday for the first time since suffering a concussion while playing special teams Oct. 7 against the Tennessee Titans.Carlson was expected to add another receiving tight end in Minnesota's multiple-tight end sets, complimenting tight end Kyle Rudolph, the team's second-leading receiver. But Carlson has only three catches for eight yards this season.Frazier said he wasn't sure if Carlson would need to be eased in after missing so much practice time."Some of it will depend on how he does this week in practice; coming off a concussion, there's some things we need to see as the week goes on," Frazier said. "But we're hoping he'll be a factor in this game. We need him. We need all the weapons we can get in a game like this. So, we're hoping we'll be able to utilize his talents, but we'll have to wait and see."Wide receiver Percy Harvin and Letroy Guion (turf toe) missed Wednesday's practice. Cornerback Antoine Winfield (knee), left tackle Matt Kalil (knee) and fullback Jerome Felton (shoulder) were limited on Wednesday.No more need for the swap: With tight end Allen Reisner promoted to the active roster before last week's game, Minnesota had an open spot on its practice squad and added defensive end Ernest Owusu on Wednesday.Owusu and offensive lineman Tyler Holmes, also currently on the practice squad, have been swapping spots on the practice squad for the past five weeks. Now the two are able to practice with the team at the same time. Reisner took the active roster spot left by Chris Cook, who was placed on injured reserve with a designation to return.Follow Brian Hall on Twitter.