If it's a ruptured or badly torn Achilles I'd say McDonnell has less chance than Lomax of advancing his career at Saints.
There are perhaps only two injuries which are more likely to end a career than cruciate ligament failure. The first is a badly dislocated shoulder. Once those shoulder balls pop out of the accompanying socket they never seem to fit back in as tightly leading inevitably to repeat dislocations down the road.
But far worse is the dreaded ruptured/snapped Achilles. That tendon is so fundamentally important to stability, speed, agility, load-balancing etc. etc. that ANY reduction in its effectiveness after repair and you're suddenly a fraction of your former self.
Recall, it was a blown Achilles which signed the death warrant to Newlove's career. He went from being one of the most fluidly agile centres the game has ever seen to someone who could only run in straight lines. Tragic, really.
If we are talking about a damaged Achilles then McDonnell's career is now hanging by a thread. If we're lucky and we get a good surgeon then he might return with 80-90% of his former skills. It's too much to think about the consequences of drawing a mediocre surgeon. However, even if we luck out and he somehow comes back unscathed - we're still talking about
months on the sidelines, with a high probability of catastrophic relapse.