Gardner - who looked off his stride from the start - was replaced by Kyle Knoyle on 40 minutes with Rovers a goal down to Cambridge.
Wellens felt he needed to call upon the 31-year-old due to absences in his midfield but admitted it may have been a mistake.
“With Ben Close with his hamstring and Tommy Rowe with his back, Dan was the only midfield player we had fit,” he said.
“He’s been out for two and a half weeks with his calf and he wanted to give it a go.
“We felt he came through training the last couple of days okay.
“I think everyone could clearly see that he was struggling.
“And the dynamics of the game helped us because it allowed Kyle Knoyle to give us the width and for Ethan Galbraith to step higher up the pitch.”
Galbraith started his second consecutive game at right back due to his excellent performance from the position in Saturday’s win over Cheltenham Town.
“Ethan played right back on Saturday and was the best player on the pitch,” Wellens said.
“He started really well again but the dynamics of the game changed because we were 1-0 down against a team that wanted to sit in.
“Instead of our winger giving us the width, Knoyley came on and did that and it allowed Rodrigo to come into the pocket of space inside.
“I thought it worked well and Kyle put in a few decent crosses in the second half.”