Getting up is not in Thor's plan. At least, not in the way Aslan likely expects. He rolls swiftly to the side, enough to get his hands and legs under him before he lunges at the Lion's shoulder, aiming high enough to hopefully use Aslan's center of gravity against him and tackle him to the ground. If it works, it'll put him in prime position to be clawed at with all four paws, unless he can wrangle the Lion's legs into a secure hold before he can recover.