Collingwood utility Ben Johnson has received some good news about his knee injury on his 29th birthday.

Johnson will be out of AFL action for about a month, after initial fears the injury might sideline him for much longer.

He had to leave the field in the second term of Saturday's one-point win over Melbourne at the MCG.

There were concerns he had suffered ligament damage to his right knee, but scans on Monday showed the only injury was kneecap displacement.

The Magpies say that will sideline him for two to four weeks.

Johnson managed 14 games last season despite a broken leg and has played 187 matches for Collingwood since his 2000 debut.

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