The former Middlesbrough midfielder was seen running sprints following the scoreless draw at Real Salt Lake after he was kept on the bench for the majority and Rennie suggests it is simply the sign of a proper professional.
"Fantastic," Rennie said of Robson's actions. "His attitude to it all and his response and the way he played, it was absolutely top-class.
"I was really pleased, and again today in training, he was brilliant."
And Robson says it was simply to keep himself sharp just in case Rennie calls on him again for Thursday's first playoff fixture with LA Galaxy.
"I'm a good professional, that's what I do," Robson told MLSsoccer.com. "I need to stay fit. If I'm not in the starting team, I go out after a game and do work - my whole career, that's what I've done.
"But the thing is, for me, I have to keep the body moving. I came on for 20, 25 minutes and it was more a bit of a run and a bit of a loosening to keep the body moving and keep myself fresh for Thursday."