The 30-year-old Australia international has been sidelined since sustaining the injury during the Socceroos' friendly against Holland on 12 October.
"It's feeling good," Grella told BBC Radio Lancashire. "The game against Manchester United may be a little bit close but I should be good for Portsmouth [on 7 November]."
Grella insists Sam Allardyce's Rovers are more than capable of embarrassing United at Old Trafford this weekend.
"It won't be an easy game because they've had a few less than positive results," said Grella. "So they'll be looking to get a winning streak back together and we'll have to make it hard for them.
"Why can't we beat them? We've beaten them before so we need to be positive and stick to the game plan."