The 5-foot-9 Atwell, who weighed 155 pounds at his pro day last month, is the lightest player drafted into the NFL since at least 2006.
"I see a player who plays a lot bigger than that," Rams coach Sean McVay said. "I've been around some of these guys that don't measure as big, but they play big."
Shortly after he was drafted, Atwell was unfazed when asked by reporters about his stature.
"I just come with a chip on my shoulder at all times, no matter was the situation is," said Atwell, who caught 140 passes for 2,307 yards and 20 touchdowns in three seasons at Louisville. "I'm a go-getter."
The Rams signed veteran receiver DeSean Jackson to a one-year, $4.5 million contract in free agency but entered the draft with the priority of adding another speedy, deep-threat receiver.
Atwell is expected to be a reliable target on intermediate and deep routes, and McVay says the rookie also has some similarities to Jackson.
"The ability to stretch the top shelf of the coverage, I think he really does an excellent job of tracking the ball effortlessly down the field," McVay said, adding that was among traits that make Jackson "so special."