Sadly, I guess you get safely bet against him.
You can't sort out fractures or damaged tendons in a foot. It's never gonna be like before an injury...no matter whether you risk surgical repair or not. Since his foot isn't gonna get any better, his skating will continue to get worse. Since skating has always been his biggest strength, there's not a lot of hope. To actually live up to his contract, he'd need to improve in areas he failed to do so ever since entering the league such as strength on the puck, strength along the boards, checking, gap control and overall positioning against the puck.
It's not impossible of course but at the same time this would have been much easier to do at 20. He's also playing for what could well be the worst team in the NHL for years (alongside Detroit I guess). Probably not helping his motivation.
Again, I'm willing to believe that the team medics talked to the Front Office about the injured foot and that the rather negative outlook was part of the reason why Karlsson isn't with the Sens anymore.