DeCastro injured his knee in the third preseason game, was placed on Injured Reserve, but was designated for return and was activated Nov. 26.
He dressed and played on the placement teams the last two weeks, but Tomlin will put him at right guard in practice and "watch him prepare during the course of the week, and if we like what we see he has an opportunity to play."
"Obviously, if a guy like (Jay) Ratliff is playing (nose tackle for Dallas), it would behoove us to have a guy like Maurkice Pouncey at the center position," Tomlin said.
DeCastro, 6-5, 316, would give the Steelers another fleet-footed lineman who utilizes great leverage in his play, but he won't be the road-grader Colon has been in the run game.
The Steelers' other top run-blocker, Mike Adams, has again been ruled out with his sprained ankle. A starting right tackle for six games after original starter Marcus Gilbert suffered a season-ending injury, Adams will miss his third consecutive game Sunday. Tomlin did not say when Adams will return.
Tomlin also ruled cornerback Ike Taylor out for his second consecutive game with an ankle/fibula fracture, and also expressed doubt about Taylor's replacement last Sunday, Cortez Allen, playing in Dallas because of a hip flexor injury.
The other starting cornerback, Keenan Lewis, also has a hip flexor injury, but Tomlin expects Lewis "to perk up" as the week unfolds. If Allen can't play, the Steelers are left with Curtis Brown, who was banished from his nickel role in the second half last Sunday; Josh Victorian, who replaced Brown and was scorched for a touchdown by Danario Alexander; and DeMarcus Van Dyke, who started 4 games and played in 14 last season as a rookie with the Oakland Raiders.
"Obviously we're running short at the cornerback position," Tomlin said. "We'll continue to work with those guys and build a plan around what they're capable of executing and executing at a high level. More than anything it's not about what we call, it's about what we're capable of executing. We'll have that in mind in regards to some of those young corners."
On a positive note, Tomlin said "the arrow is pointing up" for Troy Polamalu, who was clearly active and effective last Sunday; and that outside linebacker LaMarr Woodley will be back on the practice field, "hopefully moving toward participation."