Running back Mike Thomas practiced at full speed in Redskin drills today, despite a hamstring injury that has sidelined him during the preseason.

But while Thomas was returning to action, wide receiver Frank Grant suffered a similar injury as he pulled up without being touched during a seven-on-seven drill.

The seriousness of Grant's injury and how much time he will miss will not be determined until Tuesday morning. If it is a cramp, he probably can resume practice immediately. But if it is an aggravation of the muscle strain he suffered last week, he could miss more time.

Trainer Bubba Tyer labeled Grant as "doubtful" for Thursday night's exhibition game against the Kansas City Chiefs at RFK Stadium.

Thomas, who was a two-day holdout before reporting to camp and eventually signing a one-year $110,000 contract, reported that he felt good today. He said that he may play, a series against the Chiefs "although I doubt it."

After practice, coach George Allen said, "On the basis of today's practice, Mike will play Thursday night."

But there is absolutely no rush on Thomas' part to get back into the lineup.

Thomas starting his third NFL season, said he would need only two exhibition games to be ready for the Sept. 18 regular-season opener against the New York Giants. That would be following a form established by Larry Brown, a team's last standout tailback who retired this season.

Thomas' main concern he said, is getting back into condition. He was winded following the 2-hour 15-minute practice. As late as Thursday, he had been unable to accelerate on his left leg and therefore is not in top shape.

"I'm tired," he said. "The leg doesn't feel too bad, surprisingly. I'm happy with today."

Thomas' return meant the demotion of Calvin Hill, who played well Saturday night against Miami, to the second-team status he dreaded last season. Allen has said that Hill, who rushed only 79 times for 301 yards a year ago, would see more action in 1977.

It is a move with which Thomas agrees.

"Twenty times a game, that's almost right for me," said Thomas. "Oh, yeah, you know that. Thirty times a game is too much for anybody."

Thomas was not the only Redskin standout who returned to drills today.

Punt returner Eddie Brown, whose 646 return yards last season was second-best in NFL history, took over as No. I from Larry Jones and will play Thursday night if he doesn't aggravate the groin injury that has sidelined him for two weeks.

Meanwhile the presence of Jim Curzi, the team's new conditioning coach, was evident throughout the drills as he worked with Thomas, Hill and tight end Jean Fugett, who have all had muscle problems.

"You try to keep them stretching all the time," said Curzi. "you want to stretch as much as possible when they're on the ballfield for three hours."

Although trainer Tyer was not as sure, Curzi insisted that Grant's injury was a cramp, not a strain, and that he would be able to resume workouts quickly.

"It just felt like a cramp or something when I was going up for the ball," said Grant. "But I won't know for sure until I walk on it tomorrow.?

Bill Brundige jogged but did not participate in the major drills. The defensive tackle has a slight groin injury, but should be able to play Thursday. With Perry Brooks sidelined with knee and ankle injuries from the Miami game, the Redskins have only two healthy defensive tackles in camp. So ends Karl Lorch and Dennis Johnson got some work at tackle today, in case of an emergency . . . Brooks' examination by Dr. Stanford Lavine in Washington was putt off a day because the team physician was busy in surgery yesterday . . . A team tickets remain for Thursday's game . . . Allen said he will use only two quarterbacks against the Chiefs. Billy Kilmer probably will play at least a half, with Joe Theismann finishing up . . . Although it is only an exhibition game, Allen is preaching revenge for a pair of 1976 losses to motivate his team Thursday . . . The revolving door for cornerback tryouts did not open today, although personnel director Tim Temerario said additional prospects may be in camp Tuesday.