The 22-year-old's previous deal at the San Mames still had two years left to run before it expired.
However, he has now agreed to extend his terms until the summer of 2019, with a buyout clause of £31.5m also included.
The playmaker, who has risen through the ranks at Bilbao, is currently undergoing rehabilitation on the cruciate knee ligament injury that he suffered late last season.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Real Madrid | 31 | 24 | 6 | 1 | 67 | 20 | 47 | 78 |
| 2 | Barcelona | 31 | 21 | 7 | 3 | 62 | 34 | 28 | 70 |
| 3 | GironaGirona | 31 | 20 | 5 | 6 | 63 | 39 | 24 | 65 |
| 4 | Atletico MadridAtletico | 31 | 19 | 4 | 8 | 59 | 36 | 23 | 61 |
| 5 | Athletic Bilbao | 31 | 16 | 9 | 6 | 51 | 29 | 22 | 57 |
| 6 | Real Sociedad | 31 | 13 | 11 | 7 | 45 | 33 | 12 | 50 |
| 7 | Valencia | 31 | 13 | 8 | 10 | 34 | 32 | 2 | 47 |
| 8 | Real BetisBetis | 31 | 11 | 12 | 8 | 38 | 37 | 1 | 45 |
| 9 | Villarreal | 31 | 10 | 9 | 12 | 49 | 54 | -5 | 39 |
| 10 | Getafe | 31 | 9 | 12 | 10 | 37 | 43 | -6 | 39 |
| 11 | Osasuna | 31 | 11 | 6 | 14 | 36 | 44 | -8 | 39 |
| 12 | Las PalmasLas Palmas | 31 | 10 | 7 | 14 | 29 | 35 | -6 | 37 |
| 13 | Sevilla | 31 | 8 | 10 | 13 | 39 | 44 | -5 | 34 |
| 14 | AlavesAlaves | 31 | 8 | 8 | 15 | 26 | 38 | -12 | 32 |
| 15 | Mallorca | 31 | 6 | 13 | 12 | 25 | 36 | -11 | 31 |
| 16 | Rayo Vallecano | 31 | 6 | 13 | 12 | 25 | 38 | -13 | 31 |
| 17 | Celta Vigo | 31 | 6 | 10 | 15 | 33 | 46 | -13 | 28 |
| 18 | CadizCadiz | 31 | 4 | 13 | 14 | 21 | 41 | -20 | 25 |
| 19 | Granada | 31 | 3 | 8 | 20 | 32 | 60 | -28 | 17 |
| 20 | Almeria | 31 | 1 | 11 | 19 | 30 | 62 | -32 | 14 |
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