Former Spanish national team goalkeeper will miss the start of the Premier League season, despite returning to Middlesbrough's training camp.
The player broke his foot in June and will not be able to help Middlesbrough from the beginning of the season.
"It is great to see Mendi back, who is eager to fight," said the new team coach Gareth Southgate on the club's website. "He missed the UEFA Cup final, but returned after a very serious injury and is determined to come back stronger than ever."
While G.Mendieta recovers and can play at full strength, Boro's first match of the season awaits, which will take place on Saturday, August 19 against Reading, who managed to finish second last season and thus qualify for the most prestigious league in England.