While Costa filled in for injured Andre Gurode in the preseason opener and could get a look at left guard, there were also two drafted rookies who started in that game for Dallas.
There is suddenly some significant youth blocking for Tony Romo.
"We're just moving guys around, trying to see what we can do," offensive line coach Hudson Houck said.
It became clear when veteran starters Leonard Davis and Marc Colombo were released before training camp that Dallas would have to depend on some young blockers. Costa was already in the mix before the Cowboys used three of their eight draft picks in April on offensive linemen.
Tyron Smith, the ninth overall pick, is already set as the starter at right tackle.
Fourth-round pick David Arkin started at left guard against Denver since Montrae Holland (back) was hurt, and still is. Kyle Kosier was switched from left to right guard so the Cowboys wouldn't have two rookies starting on the same side.
Then in practice this week, rookie center and seventh-round pick Bill Nagy got some snaps with the first-team offense at left guard.
"I've got most of them down," Romo responded with a smile when asked if he knew the names and numbers of all the youngsters.
"We obviously needed a few young guys to step up to really help this football team this year," Romo said. "Without the offseason, it's been kind of a crash course. Mistakes are going to happen. That's part of the growing curve. We obviously need them to step up quickly and they're doing a good job battling every day."
The Cowboys had the day off Wednesday. There will be workouts Thursday and Friday with the San Diego Chargers ahead of Sunday night's preseason game.
Costa made his first NFL start at left guard last season in a game when Kosier and Holland were both hurt. After Costa started last week, Jones was glowing in his review, saying the undrafted player was having one of the best preseasons so far.
"He's very good at quarterbacking, so to speak, in the offensive line. He's quick with his reads. He's clear with the rest of the offensive line, and it's a big plus," Jones said. "And then he's just, physically, as well as technique, he's just playing outstanding."
While Gurode returned to practice and immediately his spot as the first-team center in the first practice after the preseason opener, Costa has certainly made an impression.
"Since he got here, he's been kind of a bulldog," coach Jason Garrett said. "He scratches, claws, fights and battles in all areas, plays from the snap to whistle."
Houck said Costa has a better understanding of the offense and the angles the Cowboys want linemen to use.
"He reacts a little quicker because he's more confident," Houck said. "Actually, I think he's probably a little bit stronger than he was.'"
Backup quarterback Stephen McGee found out early how fanatical Costa was about practicing shotgun snaps. McGee recalled being in a meeting one day with Romo, Garrett (then only the offensive coordinator) and quarterbacks coach Wade Wilson when Costa interrupted them.
"Phil had just gotten here from Maryland, they didn't even know his name probably. ... 'Excuse me guys. McGee, you about ready to get some snaps?' We're just dying laughing," McGee said. "That's Phil Costa. He doesn't care who you are, if you're Jay Ratliff or whoever. He's going to fight his butt off. That's how he plays the game. That's how he works. He's a winner."
Costa has heard about the comments by Jones and the stories told by McGee.
While that's all nice, Costa knows he can't be content with what's being said. He has to keep performing on the field.
"Just keep working from there, and building off that," Costa said. "You have to have an edge out there to be a lineman. You have to be smart, and you have to be physical. ... Last year, you come in with more the mentality that you want to make the team, and you want to play. This year, you come in with the same idea and have high goals."