St Kilda midfielder Luke Ball has paid a heavy price for his latest act of courage during the thrilling final stages of Sunday's heartstopping win over Port Adelaide at Telstra Dome.

Ball bravely backed into a marking contest in the dying minutes - colliding heavily with his much bigger teammate Justin Koschitzke in the process - but as a result of Ball's bravery, the ball was forced to ground and Jason Gram gathered the crumbs and booted the match-sealing goal.

But Ball limped off immediately after what Saints' coach Ross Lyon described as a "massive collision" and scans on Monday revealed the former skipper has strained his left hamstring and is expected to miss the next two to three matches.

The loss of Ball could not have come at a worse time for the Saints, whose win over Port enabled them to move back into the eight at the expense of Brisbane.

But the seventh-placed Saints still need to win at least two of their last four games to play in the finals for the first time since 2006 and face a huge game on Saturday night against eighth-placed Collingwood with the loser of that match to drop out of the top eight should ninth-placed Brisbane upset third-placed Hawthorn in Tasmania early in the day.

Ball will not only miss that game but also the following week's trip to Perth to take on Fremantle and possibly the crunch round 21 game against sixth-placed Adelaide in Melbourne.

The injury means Ball may have only the round 22 clash against Essendon to get himself match fit for the finals, if the Saints make it.

The 24-year-old - who has battled numerous groin injuries during his career and has also overcome many bruising hits - had played every game so far in 2008 after managing just 18 last year when he battled osteitis pubis and he had been on course to play his first full season since 2005 when he not only shared the Saints' best and fairest award but was also named in the All-Australian side.