Valencia have reportedly protested to UEFA about the state of the San Siro turf ahead of their Champions League tie with Atalanta.
The first leg of the Round of 16 kicks off on Wednesday at 20.00 GMT.
According to Marca, coach Albert Celades and captain Dani Parejo were visibly irritated during their pitch inspection.
It is no surprise to fans of Serie A that the turf is not in ideal condition, as the Stadio Giuseppe Meazza pitch has been a problem for many years.
The recent schedule seems to have made the situation worse, as last week it was used twice in 48 hours for the Coppa Italia semi-final featuring Inter and Milan on Wednesday and Thursday.
Milan then played a Serie A match with Torino on Monday evening.
Atalanta have played all their Champions League home fixtures at San Siro because the Gewiss Stadium in Bergamo is undergoing restructuring work and does not yet meet UEFA standards.
A reported 43,000 Atalanta fans are making the journey from Bergamo to nearby Milan for the historic match.
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