The stern 59-year-old coach had previously expressed concerns over the Brazilian's physical shape and was thus delighted by the midfielder's energy in training
The playing staff of the Wolves were being put through their paces on the athletic track in order to monitor their lactic acid levels whilst endurance running.
After the test was complete the players then had to do a few more laps, and it was here that Diego caught the eye of Magath with his improved stamina.
"I was not expecting it, this has surprised me too. Diego was strong," Magath said to Wolfsburger Allgemeine.
Yet the Brazilian was not the only one to please the coach with Ivica Olic, Emanuel Pogatetz and Yohandry Orozco also giving their all.
"Most of them have made a good impression. Some were even above average. But in others it is easy to see that they are not at that level."
Diego had been heavily linked with a permanent move away from the Bundesliga club after spending last season on loan at Europa League winners Atletico Madrid, but will have done his future at Wolfsburg no harm by alleviating the coach's fears over his physical condition.