However, UEFA have measured the grass and confirmed it was 26 millimetres, comfortably within the permitted limit of 30 millimetres.
Arsenal staff were seen on camera measuring the surface during their pre-match checks and remonstrating with UEFA officials, believing the grass was longer than regulations allow and had been left deliberately long to slow their passing game. The complaint was formally submitted to UEFA, who conducted their own inspection.
Spanish journalist Guillem Balague reported the outcome directly: the pitch measured 26mm — the same height it had been for Atletico's quarter-final against Barcelona, who had raised similar concerns about the surface earlier in the tournament.
Atletico Madrid have history with these complaints. Tottenham raised concerns about heavy watering before their last-16 tie earlier this season. Barcelona questioned the grass length during their quarter-final visit. Both clubs left without satisfaction. Arsenal are now the third team this campaign to dispute conditions at the Metropolitano.
UEFA's Article 34 sets out clear requirements: the grass must not exceed 30mm, must be cut to a uniform height across the entire surface, and pitch watering must be applied evenly rather than targeted at specific areas. Arsenal's concern about selective watering at half-time was also noted but fell within allowable parameters under the regulations.
The game finished 1-1, with Viktor Gyokeres converting a penalty to earn Arsenal a draw they take into the second leg at the Emirates.
Arsenal could still submit a written complaint if they choose to do so, but with UEFA having already cleared Atletico, the avenue for any formal sanction appears narrow.