Running back Mike Thomas, less than 30 minutes into his first football practice of the season, pulled up lame today with a slightly strained hamstring in his left leg. And George Allen couldn't resist saying, "I told you so."

Thomas had held out for two days along with wide receiver Frank Grant before both reported to camp Tuesday night. "As soon as a guy reports late he gets hurt," Allen grumbled. "It hurts the player personally, it hurts the team and it hurts the organization. Now he'll fall even farther behind."

Thomas' injury was not considered serious and he went immediately to the training room for ice treatments. He dressed for the afternoon practice, but did not participate.

"I just hope it's not too serious," Allen said. "Hamstrings are deceiving because even when they heal, you worry about them . . . Yes, I think it's a pull. We'll just stretch him and have him take treatment on it . . . I've seen these things take months (to heal) or two or three days."

Thomas said he felt his leg "sort of cramp up" as he was walking back toward a drill line. "It's just a little stiff right now," he said, "but I don't think it's anything serious. You know me. I'll be back out there quick enough."

All in all, it has been a rocky 10 days for most of the Redskins running backs. Fullback Eddie Moss was sidelined for three days last week from heat exhaustion. Rookie tailback Mike Northington walked out of camp last week and hasn't come back.

James Sykes, another rookie tailback, twisted his knee last week and is playing while heavily taped. Larry Brown hurt his foot Monday and announced his retirement Tuesday. And fullback John Riggins has announced he has a speech problem, telling reporters requesting interviews. "It's nothing personal, but I just don't have anything to say."

The same cannot be said for the man who would like to beat Riggins out of his job, a longshot possibility at best, but not the way Willie Spencer talks. "If I'm given the chance and the playing time, I can be one of the best backs in the league," he says.

Spencer, it may be recalled, was the big back the Redskins coveted after the World Football League folded in 1975. Spencer was actually at Redskin Park, his pen poised to sign with the team, when NFL commissioner Pete Rozelle ruled that WLF players would not be allowed to play the rest of that season.

Spencer, 24, later had second thoughts about playing for the Redskins, and, after signing with the Minnesota Vikings, said he spurned Washington because he thought Allen would run him into the ground "just like they did with Larry Brown."

Things went badly for Spencer in Minnesota. He had a motorcycle accident six weeks before the start of the Viking training camp, reported out of shape and overweight and was placed on waivers midway through the season.

"Willie never had a chance here," Viking general manager Mike Lynn said today. "He was in that accident, and he was never really 100 per cent. At the start of the year, we had a lot of close games, and we couldn't get him in there. Then we needed help at another position and we had a lot of backs, so we let him go.

"School is still out on this guy. He's still a mystery. I do think this will be his last chance. If he doesn't make it now, I don't know if he'll ever make it."

Lynn refused to comment on reports that Spencer was somewhat of a problem child in Minnesota, that he occasionally showed up late for practices and meetings. "I don't remember that being a major problem," Lynn said. But Spencer does.

"Yeah, it might have been so," Spencer said. "I had a lot of problems there - the accident, then wanting to play and not getting a chance. I just really couldn't concentrate on football. Things just didn't work out like I thought they would."

The Redskins became slightly leary of Spencer last winter, when he showed up at Redskin Park for one of Allen's mini-camps weighing 250 pounds. He also immediately established his presence by saying he felt he was just as good as Riggins, if not better.

"Yeah, I did say I was better. I don't regret anything I've said in the past," he said. "We'll just have to wait and see and let the people decide."

The - people particularly - the coaches will get a chance to do that Thursday when Spencer and all the other young backs take part in a rookie scrimmage against the Eagles at Hershey, Pa., starting at 4 p.m.

"I'm looking forward to carrying the football," said Spencer. "They never let me do that in Minnesota. Going there was probably one of the biggest mistakes of my life."

Allen, meanwhile, already has decided he wants Spencer to play between 230 and 235 pounds this year. After three days of practice, Spencer has trimmed down to 238.

"People don't realize it, but when I was in the WFL I played at 245," he said. "I can carry that weight, but if they want we lighter, I'll do whatever they say."

Spencer, who drives a sleek powder-blue Corvette around town, did himself no favors when he failed to show up for rookie week. Allen groused about his absence almost daily.

"I just had some personal problems I had to clear up," Spencer said. "I'd just like to leave it that."

Allen was asked this afternoon how Spencer has looked after three days. Before he answered, he made a major production of finding a chair so he could "knock on wood" and say, "He looks too good."

"He's a great athlete, he's catching the ball and he's quick. He looks like one of our offensive tackles who's playing fullback."

Spencer, of course, probably would agree.

Allen said he probably would take 48 players to Hershey for the scrimmage, with most veterans left behind for a separate workout. In addition to a regular 60-play scrimmage, the teams will have seven-on-seven, nine-on-seven punt-protection and field-goal drills . . . Wide receiver Bill Bryant was smacked hard after making a spiffy catch in the morning practice and suffered a badly bruised back . . . Allen said the offensive line is "in the best shape they've been in all the years we've been here" . . . there has been no progress in getting offensive tackle Tim Stokes to camp.