The reigning Brownlow medallist was substituted out of the game in the first quarter after receiving a heavy but fair bump from Power big man Justin Westhoff.
The Bombers captain sat on the interchange bench with his left arm in a sling for the remainder of his side's 18.10 (118) to 12.16 (88) win at Etihad Stadium on Sunday, which strengthened the Bombers' hold on a top-four position.
Watson told Fox Footy after the game that the initial prognosis was four weeks on the sidelines.
The Power's fast, attacking and creative game under coach Ken Hinkley challenged the Bombers in the second half but they withstood the pressure without the influence of Watson, who was considered a favourite to win back-to-back Brownlows before the injury.
Others stood up in his absence.
David Zaharakis ran with the ball and gave some drive. Jake Melksham's good form continued and he looked more confident and composed with the ball in his hands.
Cale Hooker cut off and shut down seemingly most of Port's forward entries, and finished with 30 possessions and 13 marks.
Tom Bellchambers kicked five goals in an impressive display and Dyson Heppell continued to get the ball and do good things with it.
All were contributors in the Bombers' six-goal-to-one second quarter, but Port pressed there after.
Through Brad Ebert, who finished with a game-high 36 disposals and 12 inside-50 entries, Port took control of the stoppages, and then general play.
Former Bomber Angus Monfries also helped to spark the Power into action, kicking three goals in 11 minutes to cut the deficit to seven points. He roved packs, snapped kicks and proved difficult to contain.
Essendon steadied, but the Power were never out of it, one of the defining characteristics of their 2013 campaign.
Hamish Hartlett kicked two beautiful goals, Robbie Gray was dangerous and emerging star Chad Wingard won a few one-on-ones against veteran Dustin Fletcher.
The Power scored five consecutive behinds to draw within 12 points with five minutes to play, before Brendon Goddard capped the win with a left-foot snap, leaving Bellchambers to boot the final two goals of the game to grab a career-best haul.
ESSENDON 3.2 9.5 13.7 18.10 (118)
PORT ADELAIDE 4.3 5.4 9.7 12.16 (88)
GOALS
Essendon: Bellchambers 5,Winderlich 3, Hurley 3, Stanton 2, Zaharakis 2, Ryder, Howlett, Goddard
Port Adelaide: Monfries 4, Gray 3, Wingard 2, Hartlett 2, Trengove
BEST
Essendon: Hooker, Zaharakis, Melksham, Goddard, Heppell, Bellchambers
Port Adelaide: Ebert, Monfries, Wingard, Hartlett, Westhoff, Boak
INJURIES
Essendon: Watson (shoulder), Stanton (leg)
Port Adelaide: Nil
SUBSTITUTES
Essendon: Travis Colyer replaced Jobe Watson in the first quarter
Port Adelaide: Paul Stewart replaced Jack Hombsch in the third quarter
Reports: Nil
Umpires: Dalgleish, Rosebury, Kamolins
Official crowd: 40,817 at Etihad Stadium
