The best thing that could happen to the Redskins this season - other then a body transplant for Billy Kilmer - is for Mike Thomas to play out his option. As Bobby Mitchell put it: "there's no way we can lose on this."

There are two ways they can lose, of course, one of which is injury, though Thomas hardly dwells on that as he seeks to establish his worth on the NFL free-agent market. He needs a banner season, probably 1,300 yards rushing at least, and if he gets it both he and the Redskins will be huge winners.

Thomas needs that sort of number to keep his name in the same breath as Terry Metcalf, who ran away from the Cardinals to the rich fields of Canada last year. Unlike Metcalf, Thomas has a reputation of slipping off the field at the first whiff of pain in addition to being rather small for a runner, as is Metcalf.

It is fashionable throughout major league sports to assume that off-the-field pressures of playing out an option will somehow deter from on-the-field performances. Fans and coaches seem to fret about this sort of maneuver. In fact, every team ought to encourage at least one or two important players to take the gamble.

Uncertain as the free-agent waters still are, history in baseball and football indicates that a player performs at least as well during his option year as he does after signing a long-term contract with two commas.

In baseball, Gray Matthews was slightly less effective his first year after free-agentry, as were Gene Tenace and the injury-prone Joe Rudi and Bobby Grich. Dave Cash, Bert Campaneris, Don Baylor, Richie Hebner and Sal Bando were not significantly better the year after gaining their freedom.

Reggie Jackson improved as much as anyone. The year he became a millionaire he was up 27 more times and scored 9 more runs, got 12 more hits, 5 more doubles, 5 more homers and 9 more runs batted in.

Of the pitchers, Doyle Alexander's won-lost percentage was up, but so was his earned-run average. Wayne Garland was dramatically worse and Bill Campbell, Rollie Fingers and Don Gullett were as impressive as ever.

The brief free-agent experience in the NFL has been nearly the same. Unlike the Yankees, nobody bought a championship with free agents. The Redskins tried, but only Jean Fugett of the trio of expensive acquisitions performed up to the standards his salary seemed to demand in 1976.

And Fugett actually caught 11 fewer passes for 154 fewer yards his first year with the Redskins than his final year with the Dallas Cowboys. Calvin Hill carried the ball half as many times with Washington as he did his final NFL year with Dallas, 1974.

John Riggins gained 437 fewer yards for Washington than he did with the New York Jets, though he also had 76 fewer carries.

So Thomas should be inspired this season.

In his first three seasons, Thomas has gained only 135 fewer yards than Larry Brown did his first three years with the Redskins. Now Thomas has two more games per season to display his skills, which is why 1,300 yards is a more realistic measure of running excellence than 1,000.

Should Thomas play as well as he and the team hope the second worst occurrence would be either his return in 1979 to the Redskins or the Redskins receiving at least a first-round draft choice as compensation from his new team.

If a team thought it was one swift and agile runner away from the playoffs, it might pay Thomas enough to merit two first-round draftees in return, though his size and durability would hamper such a deal.

The worst possibility for the Redskins is that Thomas, like Metcalf, would trot off to Canada, collecting a handsome salary but bringing nothing in return to the team.

For Thomas, the most depressing though would be an injury that in fact might cause him to be playing for less money this year than last despite getting an automatic 10 percent raise in his option season.

That would happen if Thomas, like Pat Fischer and Bob Brunet, were hurt so seriously this season that he was unable to pass a physical for next season. According to the collective bargaining, agreement between the NFL owners and players, anyone under Contract is entitled to half his prior season's pay up to $37,500.

For their freedom, players apparently sacrifice this injury protection, though it may take arbitration or the courts to decide the issue.

Consider the gamble Walter Payton apparently is taking with the Bears. He has turned down a reported $375,000-per-year-offer to play for $66,000 so he might possibly earn $700,000 or so in 1979.

But that $66,000 would actually be cut in half if Payton were seriously hurt this season, because he would not get the extra half-season's pay.

Still, just as though football players do not consider money the way ordinary humans to they also have a special disregard for injuries. Full speed ahead and dam the ligaments is the usual motte which the Redskins hope Thomas chooses to follow this season.