But the OP specifically asked about athletes who look out of shape. The shoe fits. And I’m real glad ol’ George never hit me. He could knock you into next week!!
In her post-Olympic days, I was amazed when Katarina Witt could do splits in the air and lift her body weight off the ice.
I always thought Mary Pierce looked too frail to be a pro tennis player.
Pete Rose always looked like he needed to put down the beer stein.
Sammo Hung, martial artist/actor.
Sammo at least looks like he can kick a speeding Mack truck off its axels.
These guys are physical specimen compared to:
Luis Tiant–Possessing one of the most distinctive windups in baseball history, he was so fat, his windup motion lasts longer than a rain delay. But he can pitch! He once kicked a batted ground ball right into his glove.
Sid Fernandez–he pitches with his left knee scraping the ground! But his fly balls stay in the park.
Yes, Terry Forster. Or, as Letterman referred to him, the “fat tub of goo”.
In the same vein, thanks Chuck, for remembering Lolich. He delineated for me the difference between ‘athlete’ and ‘baseball player’.
Rick Reuschel. Fat like Humpty-Dumpty, and I KNOW it–saw that feller in his underwear once when I got a pass into the Cubs locker room. 
Were you thinking maybe of Danny Ainge? He played a season for the Blue Jays before switching to basketball and playing for the Celtics.