Melbourne have suffered another injury a week out from the start of the AFL season.

The Demons say second-year utility Jack Grimes, who has impressed during the pre-season, will be out for up to a month with a stress injury in his lower back.

Fellow backman Daniel Bell is also out for four weeks with a broken thumb, which he suffered in Saturday's NAB Challenge loss to the Western Bulldogs.

"This is very disappointing for Jack and for the club," coach Dean Bailey said in a statement.

"Jack has completed a full pre-season and has worked incredibly hard throughout.

"We are hopeful that Jack will respond well to the treatment, and will be available for selection in three to four weeks."

Grimes, Melbourne's second selection in the 2007 draft, made his senior debut in round 22.

He also suffered a back injury last year that put him out of action for more than half the season.

Aaron Davey (heel) and Ricky Petterd (back) were also injured last Saturday and they will undergo tests before the March 29 season opener against North Melbourne.

Meanwhile, Melbourne have also confirmed they have elevated rookie ruckman Jake Spencer after putting Mark Jamar on the long-term injury list.

Jamar has a stress fracture in his foot and the Demons hope he will be available in eight weeks' time.

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