He sustained medical ligament damage after coming on as a half-time substitute in last weekend's 2-2 home draw with Aston Villa.
"Personally I am disappointed for Lucas because he's been very important for our squad this season," he said.
"It was something that was innocuous really and has put him out for a few months.
"Hopefully he'll get back sooner rather than later.
"He'd been in good form this season and had worked his way back into the Brazil squad. Knowing what he is like, he'll be back sooner rather than later - and he'll do everything in his power to get back.
"In the meantime, that leaves us short of a really good player."
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