Winger Wes Atkinson and midfielder Aaron Birch came off the bench to make their first-team debuts - while striker Kemar Roofe started for Steve Clarke's side.
"The young lads showed a lot of enthusiasm," Keen told the WBA website. "I thought Kemar did really well in the first half.
"He ran out of legs in the second half and we spoke about it afterwards. The physical demands on a Premier League player at this level are so high. But in the first half positionally in terms of the way he dealt with the ball, I thought he did really well.
"Wes Atkinson also deserves a mention.
"He got ten minutes when he didn't know he was in the squad until 3pm.
"He nearly got a goal and produced a couple of moments when he did well."