The Charlotte, N.C., native graduated high school in December and joined the Cardinals in January. He not only enrolled in school but was able to participate in spring ball with the team.
And that gave the 5-foot-11, 180-pound Alexander a huge leg up.
"He's been really impressive for a young guy," defensive coordinator Todd Grantham said. "We're still trying to work with all the young guys to understand their craft, but I've been very pleased with him so far."
A former South Carolina commitment, Alexander is likely to see a lot of time in the rotation at cornerback along with Shaq Wiggins, Trumaine Washington, Cornelius Sturghill and several others.
"I know a lot more plays and I am way ahead since I was here," Alexander said. "It was so fast for me to begin but now it's slowed down for me and I am just so happy that I came here early.
"I'm excited to be out here and be part of this team."
Alexander, a two-way star at Rocky River High School, has also been returning punts. Petrino said on media day that he could be a factor as a weapon on special teams.
"I do like the ball in my hands," Alexander said. "They have me back there and I just want to help the team."
Alexander, who ranked as the fifth-best player in North Carolina, committed to South Carolina in June, but an official visit with Louisville just before the mid-year guys who enrolled signed convinced him to flip the Cardinals.
Alexander ranked as the No. 89 corner in the country and No. 36 among corners in the South by Scout.com.
As a senior, Alexander had over 70 catches for more than 1,300 yards and 25 total touchdowns. He also had 35 tackles, four interceptions and returned a punt, two kickoffs and an interception for a TD.