Barker's first game in charge of the Red Devils on Tuesday night ended in a penalty shoot-out win over Championship Millwall following a battling 2-2 draw at The Den.
"It showed the character of the players," said Barker. "They didn't stop doing what we asked them to do, which was pass the ball and being disciplined.
"We did all of that, and to do it for two hours away at a Championship side speaks volumes for the players.
"It cemented what I thought about a few players and it's a learning curve for everyone in terms of where we want to get to."