The set-back will come as a huge knockback to coach Darren Edwards whose side clocked up their second RaboDirect PRO12 win of the season against the Gunners.
But with Lydiate set to go under the knife on Sunday and certainly missing Wales' autumn internationals Edwards was in a sombre mood.
"It's a huge blow for him and for us," he said. "I don't know the time scale yet or how long he will be out. Let's just hope he still has a chance to achieve his goals.
"Dan is a world-class player whose tackling is known all over the world. He's also a great guy to have around the environment and is a good influence on everyone around him. It will be very sad not to have that presence."
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