Yamal was forced off with an injury suffered after scoring a penalty against Celta.
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Gavi said: "He's the best and it's a real shame not to have him.
"He was upset in the locker room. We don't know how long the injury will take, but we hope it's as short as possible."
The win took Barca nine points clear at the top of the LaLiga table.
Gavi added: "It was very important to win today. It was a key victory to win LaLiga, which is not yet decided."