Jefferson was a promising young pitcher at times, and at others, he didn't even look like a major league pitcher. He had his only winning season in his rookie year with Baltimore, when he went 6-5 with a 4.11 ERA. The right-hander threw hard, but was often wild, setting a Blue Jay franchise record with nine walks given up in a game against Baltimore on June 18, 1977. In Jefferson's major league debut on June 23, 1973, he was pitching a shutout at Fenway Park against the Boston Red Sox, up until the 9th inning with 2 outs and nobody on, when Rico Petrocelli hit a home run to tie the score. The Orioles would get Jefferson the win, 2-1, with a run in the top of the 10th.

His best season probably came with the miserable expansion Blue Jay team in 1977, when he went 9-17 with a 4.31 ERA and 8 complete games. Jefferson's appearances on leaderboards were generally for bad things; he was in the top 10 in the league in walks allowed three times, top 10 in losses twice, top 10 in earned runs allowed twice, and 2nd in the league in home runs allowed in 1978, and although he always showed he had the talent, things never fell into place, and he ended his career after a brief stint with the Angels in 1981, where he went 2-4 with 3.62 ERA.

In a memorable game during the 1980 season, Jefferson, although finishing the year 5-13 with a 5.26 ERA, outdueled Oakland A's ace and eventual 22 game winner Mike Norris in extra innings. Jefferson threw his finest game as a professional, an 11 inning Complete Game Shutout, striking out 10 in the process.

In a 9-season career, he had a miserable record of 39-81, with an ERA of 4.81, in 237 games and 144 starts. Jefferson had 25 complete games in his career, with 4 shutouts. He picked up 522 strikeouts all-time, in 1085 and 2/3 innings pitched.