In his first game back since he injured his left ankle in round 11 last year – which required season-ending surgery – Mumford came off early in the second quarter and played no further part in the game at Drummoyne Oval.
GWS coach Leon Cameron confirmed Mumford's ankle had forced him off but stressed it was purely precautionary.
WATCH: Mumford's ankle 'a little bit sore'
"He started off really well but he was just a little bit sore so they (medical staff) didn't want to take any risks, and neither did I," Cameron said post-match.
"We just made sure we iced him up and hopefully over the next few days we find out a little bit more.
"It's hard to say whether it's an issue or not.
"We can't rely on Shane to be our number one ruck every week; we want that to happen but he's 29 years old and he's been battered around a fair bit.
"He's had a good pre-season, touch wood he's going to be right for next week, but time will tell."
It was the only slightly sour note for the Giants who ran the Swans ragged from the opening bounce and went on to win 0.14.10 (94) to 1.8.3 (60).
The match was delayed for 15 minutes when an electrical fault in one of the ground's six light towers short-circuited the other five, but when the action got underway both sides played with an intensity more in tune to a regular season encounter.
Friday night lights out for Swans and Giants at Drummoyne
Steve Johnson showed his recruitment might be a masterstroke for the Giants, the former Cat booting three goals and finishing the game with 17 quality possessions playing as a permanent forward.
GWS started plenty of their attacking moves from deep in defence and their ball transition was electric with the likes of Heath Shaw, Zac Williams and Nathan Wilson linking brilliantly with midfielders Callan Ward, Ryan Griffen and Dylan Shiel in particular.
Tall targets Adam Tomlinson and James Stewart were reliable targets for the winners, with star Jeremy Cameron unusually quiet for the night.
Swans onballers Josh Kennedy, Luke Parker, Tom Mitchell and Dan Hannebery found their usual share of the ball, but they found it difficult to move the footy inside their forward 50 with any system thanks to the Giants' tackling pressure through the middle of the ground.
Lance Franklin worked hard both ways for the Swans but, apart from the odd burst from Isaac Heeney and big man Callum Sinclair, he was their lone avenue in attack.
Coach John Longmire couldn't find a lot of positives in the performance after the game.
"We didn't have many winners, I think if you went through some of the players that played well you'd probably struggle to find more than a handful," he said.
"That was disappointing, we didn't do a lot right both form-wise and with the way we wanted to play."
WHAT WE LEARNED
Greater Western Sydney
The Giants could have the most attacking defence in the league led by reigning club champion Heath Shaw. GWS moved the ball at breakneck speed from their back 50 thanks to Shaw, Zac Williams and the speedy Nathan Wilson. Co-captain Phil Davis, Aidan Corr and Caleb Marchbank provided the size, bringing the ball to ground and that's where Leon Cameron's men swoop and launch many of their forward 50 entries.
The Swans need to find some outside run to match their inside ball winners. Josh Kennedy, Luke Parker, Dan Hannebery, Tom Mitchell and Kieren Jack make up one of the hardest midfields in the competition, but they're all similar types and Jack is really the only one with some leg speed. Lewis Jetta is no longer at the Swans, while Gary Rohan and Ben McGlynn are struggling with injury, meaning outside runners are few and far between.
Greater Western Sydney
Star recruit Steve Johnson started his first game in Giants' colours slowly butgot going with a brilliant second term. The three-time premiership Cat kicked three goals for the match, showed off some of his tricks and had a strong influence on the contest whenever he touched the footy.
Rookie Tom Papley showed some encouraging signs in limited ground time and kicked a textbook small forward's goal in the final term. Nic Newman was another rookie to play and looked composed in his first outing, while Jordan Dawson kicked the game's only supergoal with his booming left foot.
The Giants leave town for the second time in the NAB Challenge to take on the Brisbane Lions at Metricon Stadium on the Gold Coast to finish their campaign. The Swans will play Carlton at Etihad Stadium in their final hit-out.
Greater Western Sydney: Nil
Sydney Swans: Dawson
Greater Western Sydney: Johnson 3, Tomlinson 2, Stewart 2, Palmer, Mumford, Griffen, Greene, Kelly, Shiel, Hoskin-Elliott
Sydney Swans: Heeney 2, Papley 2,Kennedy, Sinclair, Franklin, Hannebery
Greater Western Sydney: Shaw, Shiel, Griffen, Wilson, Johnson, Davis
Sydney Swans: Parker, Mitchell, Kennedy, Grundy, Rampe, Franklin
Greater Western Sydney: Mumford (ankle)
Sydney Swans: Nil
Umpires: Leigh Fisher, Nick Foot, Ray Chamberlain, Mathew Nicholls
Official crowd: 4,754 at Drummoyne Oval