In a contest red in both tooth and claw, neither side could engineer the try that would have blown the game open and Gloucester can be thankful that Mercier matched Worcester's James Brown kick for kick.
But Ryan admitted Gloucester were fortunate to emerge unbeaten after weathering a severe bombardment from the home side.
"We were probably lucky to come out with a draw," he said. "I am certainly not happy with the performance but pleased with the two points.
"We were under no illusions about what we faced here - it was a local derby with an awful lot riding on it and that produces errors, particularly on the opening day of the season, because there is a lot of pressure on everybody.
"At times we struggled to get enough ball but we have got two points - there are a lot of very sore bodies in the dressing room after a game like that."
The major downside for Ryan was the 6th minute injury to skipper Adam Balding. He was helped from the field after a bone-crunching collision with suspected damage to a cheek bone that will be further assessed this week.
Balding was sorely missed and had been at the heart of a punishing fourth minute line-out drive that created the lead for Gloucester. Alex Brown held a towering catch, the monstrous presence of Olivier Azam helped power forward a rolling maul and when the ball came back, Mercier slotted over the drop from Peter Richards' pass.
And Gloucester regrouped from Balding's departure to go 6-0 ahead with Mercier's first penalty five minutes later.
It took Worcester fully half an hour to get into their stride but they started to pinch a good amount of line-out ball through Tim Collier and Craig Gillies and were rewarded with two Brown penalties that levelled the score after 36 minutes.
And when he knocked over his third successful attempt a minute before the break it looked as though Gloucester would be trailing at the interval.
But Worcester had not counted on Mercier. Gloucester earned a penalty fully 10 metres inside their own half and with the clock ticking down, he could have kicked for touch. But Mercier stepped up and launched a 70-yard attempt that skimmed through the air to level the scores again at 9-9.
There was plenty of tension and mistakes but the commitment was unquestionable. Both teams slugged themselves to a virtual stand-still after the break and it was left to Mercier and Brown to continue the shootout.
Brown put the home side 12-9 and then 15-12 ahead with a quarter of the match remaining but Mercier's 71st minute penalty salvaged a draw.
Both teams huffed and puffed searching for an opening but nothing much went beyond two or three phases due to the high number of mistakes or the whistle of the referee.
Worcester dominated possession and territory in the second half but were unable to unlock Gloucester's defence. Their line-out functioned well and they also scrummaged effectively but in the last 20 minutes Gloucester came close to snatching victory.
Mark Foster's dashing break and some powerful surges from Mike Tindall stretched Worcester's rearguard but on a frenetic afternoon, both teams were made to settle for two points on the opening weekend of the season.
GLOUCESTER: Jon Goodridge; Mark Foster, Mike Tindall, Henry Paul, Rob Thirlby; Ludovic Mercier, Peter Richards (Haydn Thomas 68), Patrice Collazo (Gary Powell 55), Olivier Azam (Mefin Davies 49), Phil Vickery, Adam Eustace, Jake Boer, Andy Hazell, Adam Balding (Peter Buxton 6).
Point scorers: Tries: Cons: Pens: Mercier 4/5, Drop-goals: Mercier.
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