Marsh was claimed off of waivers from the New England Patriots on Wednesday after being waived the day before. In four NFL season, he has 74 tackles, four sacks, and two forced fumbles during his 46 game appearances.
"He adds speed, for sure," Saleh said. "When you watch his tape, he's relentless. He does not stop. He's the definition of all gas no breaks. He does provide added juice off the edge. But, he also brings in a little bit of experience with pass rushing that right now Elvis [Dumervil] has from an edge-rush standpoint.
"The guys are starting to develop and learn, but he already has it through his first three years. Working with [Seattle Seahawks DL] Cliff Avril and [Seattle Seahawks DL] Michael Bennett and those guys, understanding he's got time in our system and he understands exactly what we look for from a pass rush standpoint.
"He brings added speed, he brings added knowledge, he brings added veteran presence. Even though he is a young guy – he's only 25 – we're excited to have him because he's very athletic. He does have enough speed, and he's relentless with his pursuit. Those are the things that make him a tremendous asset for us."
Marsh has familiarity with the style of defense the 49ers run. Before being traded to the Patriots in September, he spent his first three NFL seasons with the Seattle Seahawks. Defensive lineman Sheldon Day, who was claimed off of waivers from the Jacksonville Jaguars on Monday, also has experience within a similar system.
Marsh and Day's familiarity with Saleh's defense could help them see action on the football field as early as Sunday's game against Seattle.
"You still have tomorrow and Saturday to work through everything, but there's so much recall," Saleh said. "They understand our techniques. They understand exactly what we're asking them to do. So, Sheldon and him could just slide right in and start playing. It's just a matter of whether or not they can pick up the differences in verbiage. So again, just working all the way through to Sunday and see how that works out."
San Francisco expects to get defensive end Solomon Thomas (knee) back and hope to activate defensive end Tank Carradine (ankle) soon. Also practicing this week is defensive end Aaron Lynch (calf).
"The D-Line, they're starting to get healthy," Saleh said. "So, there's going to be some healthy scratches. It's going to be who's able to perform during the week. It is a constant competition. It's a good thing, I guess, because when you look at it, you're getting guys who are all very capable of playing. You've got a bunch of guys in there.
"So, the fact that we're getting healthy, and Aaron, he's out there working his tail off. Again, there's going to be a couple of guys that aren't going to be up because of numbers. It's going to be a battle."
Saleh is also excited to get Thomas back and explained what he wants to see from the rookie in the 49ers' final stretch of the season.
"Before he got hurt, he was leading all rookies for tackles for loss," Saleh explained. "He was leading all defensive linemen in tackles. He had two sacks. He was actually producing. He was playing at a very high level, especially in the run game. The big thing for him is to continue to build on that and to continue to work and develop as a pass rusher so by the end of the year he's feeling confident and we're feeling confident that he's going to be the inside pass rusher that we're all hoping he'll be."
As for Carradine, the 49ers are hopeful to have him for Sunday's game, but the coaches are comfortable putting off his return. Whether he plays or not, the 49ers must activate him off of injured reserve prior to facing the Seahawks.