Carter was hit with two criminal charges on March 1 over his alleged role in a Jan. 15 car crash -- and then just 14 days later, he showed up to his Pro Day nine pounds heavier ... before struggling during the workout.

While the 21-year-old has since struck a plea deal with prosecutors to put his legal issues behind him ... Lombardi says it all would have him seriously wary of picking the defensive lineman in next month's NFL draft if he were still in charge of a team's war room.

"A lot of concern," the former Cleveland Browns general manager said. "You want this guy to become your team captain. When you pick a player like this in the top 10, you want him to become Richard Seymour. You want him to become a Hall of Fame player. You want to hit."

Of course, Carter had been considered a top-five pick right after the college season concluded -- and maybe even the No. 1 overall selection -- but now, Lombardi's making it clear ... top-10 might be out of the question.

Lombardi says Carter's jump from 314 pounds to 323 pounds in just two weeks is particularly alarming, noting, "If your weight starts to zoom up because you're literally sitting on the couch, that's a problem."

"He's got a lot of questions to answer."