"It's a big blow again," said Milner. "He's come through injury problems and worked so hard to get fit.
"He showed his class when he came back, even though there were a few doubters. It's unbelievable there could be doubters after the quality he has shown over the years and the goals he's scored.
"But he's mentally strong and he will work twice as hard to get back fit again.
"You need to be mentally strong when you're in the gym on your own in the afternoons - it can be tough. I'm sure he'll be scoring for Newcastle and England again soon."