The 20-year-old scored twice late on in Newcastle to secure the win for her side but while scoring her second, in the second minute of injury time, she collided with Jets goalkeeper Izzy Nino.
McNamara, who has suffered two ACL ruptures in her short career, was in tears as she was carried off the field. She had been called up to the Matildas squad this week to play two friendlies against Canada in December.
In Brisbane, new Roar coach Alex Smith had a losing start to his tenure after Western United prevailed 1-0 at Ballymore. United's Matildas representative Chloe Logarzo set up the winner with a curling corner that was latched onto by Kahli Johnson.
The match of the day, however, was in Newcastle, where Matildas star Emily van Egmond returned to her home club on a four-game guest stint. The accomplished midfielder threatened to steal the match with a brilliant showing. She scored the opening goal after a sensational interchange of passing with striker Lara Gooch.
Her through-ball from halfway to set up Gooch for the second was sublime and gave the Jets a 2-1 half-time lead. It looked like being enough until McNamara struck in the 78th minute and again in injury time to break Newcastle hearts and ensure Melbourne City stayed undefeated after five rounds.
Worts was the shining light in Adelaide. The 27-year-old from England scored her brace in the first half with composed finishes to continue her early season goal-scoring form for the defending champions.
The Sky Blues set up the win in the first half, when they dominated scoring opportunities. Defender Tori Tumeth went on a brilliant run down right and cut back inside to Worts who finished with precision in the 20th minute. Six minutes later, full-back Abbey Lemon surged down the left and delivered a pinpoint cross. Worts timed her run to perfection and headed home.
New Zealand international Hannah Blake scored three minutes into first-half stoppage time for Adelaide, despite protestations from Sydney players for offside. But Sydney captain Princess Ibini sealed the result via a penalty after she went on a scheming run into the box and was brought down.
It was a big day for veteran Jessica Seaman who was starting her first A-League Women game for Sydney since Jan. 11, 2009. The afternoon, however, belonged to Worts, who had made 39 appearances for Adelaide United in three seasons, scoring 18 goals.
The Sky Blues moved to six competition points and seven behind leaders Perth Glory but have two games in hand due to their recent week in Uzbekistan for the AFC Champions League. Adelaide United have one point from five matches.