Wirtz made two assists in the midweek Champions League win at Eintracht Franktfurt - his first meaningful contributions since joining from Bayer Leverkusen.
Robertson, 31, said: "I thought he was outstanding, excellent (against Frankfurt). I thought he grew and grew into the game, especially in the second half.
"The players around him helped him a lot - Curtis Jones was outstanding - and I thought the three midfielders helped us get the momentum and keep the ball.
"Flo has been really good since he came here. People focus on the number of goals and assists, but sometimes it's unfair because he has created an incredible number of chances and on Wednesday some of the chances he created ended up as assists. Hopefully he can take a lot from this performance."