The coach of the 2009 German champions demands the Brazilian to work harder, having been left underwhelmed by the midfielder's first pre-season session back with his parent club
The 27-year-old spent last season on loan at Atletico Madrid and has been heavily linked with a return to the Liga side on a permanent basis this summer, while Flamengo are also purported to be in the running for his signature.
However, no firm offer is presently on the table for Diego, and after returning from a break in his homeland, his strained relationship with Magath was evident during his first training session back at Wolfsburg on Thursday.
Acccording to Bild newspaper, Magath approached reporters watching Diego's thirty-minute stint on the training pitch and asked: "Did you see him willing to work? Because I did not."
Unaware of his trainer's comments, Diego was pleased with his efforts, but refused to be drawn on his future.
"For my first training in a while, I think I've done well," he maintained.
"My future? There is still nothing to announce. For now, I do not want to talk about it."
The next 12 days are likely to determine whether the playmaker leaves the German club, as Magath previously stated that any deal for the player must be done by August 1.
Earlier in the summer, AS had also reported that Diego vowed never to play again in the Bundesliga.