It's hard to call any season that Tom Seaver has an off season, especially one in which he wins 21 games, but 1972 might have been it. Seaver put up some decent numbers, like the 21 wins and the 249 strikeouts in 262 innings, but he also allowed 10.20 baserunners per nine innings and he lost 12 games. On a support neutral basis, Tom Seaver would have been an unimpressive (for him) 18-15.
The Mets had their fourth winning season in a row but they fell well short of winning their division. The Pittsburgh Pirates dominated the National League East with a 96-59 record in the strike shortened season and the Mets finished 13 1/3 games off the mark despite finishing 10 games above .500. Ironically, they'd win the division the next year despite having one less win.
Despite the down season, Seaver did have some great moments during the season. Pitching in the front end of an Independence Day double header, Seaver pitched a one hit shutout in which he struck out 11 and walked walked four. That one hit didn't come until one out in the top of the ninth. On September 20, he gave up one run on five hits, but he struck out a season high fifteen batters. Nine days later, on September 29, he threw a two hitter in which he struck out thirteen.
Here's the numbers....
Wins 21
Losses 12
Games 35
Games Started 35
Complete Games 13
Innings Pitched 262
Hits 215
Runs 92
Earned Runs 85
Walks 77
Strikeouts 249
ERA 2.92
Runs Saved Above Average 10
Shutouts 3
H/9 7.39
BR/9 10.20
SO/9 8.55
BB/9 2.65
SO/BB 3.23
Neutral Wins 18
Neutral Losses 15