And the in-form forward could miss several key fixtures in the coming weeks as he battles with a side problem.
"He's got a strain on his side, something he picked up on Tuesday," said Richardson.
"We'll take it day by day, but obviously the medical staff are all over it.
"You'll have to ask the doctor exactly what the problem is, because I'll get it wrong and he'll be on to me!
"He's obviously a big miss, but fingers crossed it's only weeks as opposed to anything more serious."
Lang has scored 11 goals for Latics this term - three short of last season's effort, which was a personal best.
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