Bell saved 43 games and had a 2.44 ERA for the Padres in 2011 before signing a three-year, $27 million deal with the Marlins. It was a bad fit from the start, as the 35-year-old compiled a 5.37 ERA and feuded with former Miami manager Ozzie Guillen.
While watching him last year, Towers said he noticed Bell's stride was a little long, which could have caused him to elevate his pitches.
"I think his stuff is the same, it's just pitching is tough and a lot of it is mental," Towers said. "You get off to a tough start, man, and you're in a new place with a big contract and high expectations, and it didn't sound he and the manager hit it off all that well. Just not a good environment."
Bell is expected to pitch in a seventh-inning setup role for the D-backs, with he and David Hernandez filling in as closer if J.J. Putz is unavailable.