Although the Philadelphia Eagles now are the only unbeaten, untied team in the National Football League, with a 5-0 record, they still have a major concern at a very important position.

At the moment, Coach Dick Vermeil's No. 1 fullback is Hubert Oliver, a 10th-round draft choice who played extensively for the first time in Monday night's 16-13 defeat of visiting Atlanta.

When incumbent starter Leroy Harris suffered a double fracture of his forearm during the exhibition season and his backup, Perry Harrington, broke his leg last week against the Redskins, Vermeil had no one left except Oliver.

To add some experience at the position, Vermeil signed free agents Steve Atkins, a former Maryland star released by Green Bay, and Booker Russell, a three-year veteran dropped by San Diego in August.

Neither of the castoffs was adequately prepared for the Falcons, of course, so Oliver, still bothered by a pulled hamstring muscle, got the starting assignment. He responded by leading the Eagles with 68 yards in 10 carries.

The 68 yards were misleading, however, because he gained 39 on the next-to-last play of the game. The third-and-10 call obviously caught Atlanta by surprise, and was an important play because it allowed the Eagles to maintain possession and run out the clock.

"The hole was just there," said the 210-pounder from Arizona. "At first, I was kinda surprised. I didn't think I'd get out in the open field. They had been jamming us up all night."

The Eagles haven't had much success running against Atlanta. In the previous three games, all losses, Philadelphia was limited to 53, 57 and 86 yards. Without Oliver's last-minute run, it would have been 68 Monday night.

"They were just stuffing our run the whole night," said quarterback Ron Jaworski. "They've always been successful stopping our run. It didn't make any difference that Hubie was playing instead of a more experienced fullback."

The principal reason the Eagles are the only unbeaten and untied team in the NFL is that they have allowed only 53 points in five games. If they are to continue their success, however, they must find some help for their No. 1 ground gainer, Wilbert Montgomery.

"Sure, they were keying on me," Montgomery said after being limited to 44 yards in 12 carries. "I expected that. It'll probably be like that all year."

Aside from his one long run, Oliver did little to distinguish himself before a national television audience. He gained 29 yards in his other nine carries and dropped the only two passes thrown to him.

The Falcons had just reduced a 13-0 deficit to 13-10 midway through the third quarter. Jaworski floated two passes in a row to Oliver, out in the flat, and both slipped through the rookie's fingers.

"Coach Vermeil didn't say anything to me after those passes," Oliver said. "It was just lack of concentration. This was my first start and all. I was a little nervous. I was the new guy in there. Wilbert and Ron were talking to me a lot and that helped. It means a lot to have people talking to you and trying to help you."

Vermeil admitted he wasn't satisfied with the Eagles' running attack, particularly on first down, but he was careful not to criticize Oliver.

"Hubie did pretty well out there," the coach said. "He's just getting over a pulled hamstring and I didn't know how long he could play. But he had something left at the end when we needed it."