Tom Seaver, the greatest pitcher in New York Mets history, joined the team yesterday for the third time.

In an attempt to strengthen a starting rotation crippled by injuries, the Mets turned to Seaver, 42, although he has not pitched since last August when he suffered a knee injury requiring arthroscopic surgery while with the Boston Red Sox. The 311-game winner is expected to need about a month to be ready to pitch in the major leagues.

He has agreed to contract terms but won't be signed until he's ready to pitch, the Mets said. He threw yesterday for the sixth time this year -- all since May 25 -- and said he could start pitching in the National League "tentatively" June 20, assuming inclement weather or injuries don't hinder his workouts.

"The need for pitching is certainly the reason I'm here," Seaver said at a Shea Stadium news conference. "I hope in the next two weeks, I could show that I can still pitch and help this club. If I feel if I can't help this ball club in a positive manner, I am going to retire."

The agreement is believed to be worth $400,000, guaranteed, for the rest of the season and to include a 1988 renewal with a buyout option for the club at the end of this year. In the offseason, Seaver rejected a $610,000 offer from Boston for the 1987 season.