Faneca points out that celebrities like Rob are constantly out at fancy restaurants and catered events -- and it's tough to maintain a strict diet when faced with so much temptation.
Rob Kardashian has an NFL sympathizer ... a former lineman who tells TMZ Sports it can be HARDER to shed the extra lbs when you have the world at your fingertips ... but he insists, it CAN be done.
We spoke with former Pittsburgh Steelers offensive lineman Alan Faneca ... who famously dropped more than 100 pounds after retiring from pro football ... and he tells us he knows the struggle big stars like Rob face when they try to lose weight.
"It makes it harder ... it takes a lot of willpower to go to amazing restaurants and order grilled fish and vegetables. It takes A LOT of willpower."
"You're at a restaurant ... and there are 100 dishes that sound great ... and you have to pick the one that doesn't."
Alan also says Rob's plan on getting back to the gym when he "chooses" is a bad excuse -- saying, "We HAVE to be in shape. We don't all need 6-packs, but you only have ONE body. And you need to take care of it. It's gonna keep you here."