But the veteran shot-stopper is determined to keep his Firhill glove rival out in the cold in the weeks to come as he targets an extended Jags first-team run.
After Fox picked up a knock in training ahead of Thistle's SPFL Premiership clash with Kilmarnock last month, Gallacher stepped in between the sticks for the 1-1 draw with Allan Johnston's side.
He retained his place for the games with St Johnstone, St Mirren and Dundee United and is set to keep the Jags gloves for the trip to Motherwell this weekend.
In his capacity as goalie coach at Firhill, Gallacher helped Fox win a place in Gordon Strachan's last Scotland squad, but the former national keeper is eager to keep his protege on the sidelines.
He said: "The injury popped up for Foxy so I had to be ready to come in and perform and look to seize my chance.
"Hopefully I have given the gaffer a headache with my performances and done enough to merit a run in the team.
"Foxy has played well this season. He is under more pressure now because people are looking at him not just as a Partick Thistle goalkeeper but a Scotland squad goalkeeper as wel, so he has a lot more on his shoulders.
"I have been there myself, but Foxy has coped with it well. He does take a bit of stick from the boys and the banter is good.
"In the bigger picture, the supporters look at him as a Scotland keeper. He picked up his injury and it is up to me to keep my chance.
"I think I have done okay and made some decent saves. Results haven't been too bad recently, but we know they could have been a lot better.
"It would have been even better if we could have got that result at home.
"It was a decent comeback against United and we showed a bit of character to get a point."
While Gallacher has performed admirably between the sticks since claiming the No.1 jersey, it has proven to be a frustrating spell for Alan Archibald's side as they have struggled to put points on the board.
The Jags return to top-flight action with a trip to Fir Park on Saturday as they look to give their survival bid a timely boost.
And Gallacher knows Thistle will have to up their game from their last clash with Stuart McCall's side.
He said: "The break gave the guys who have been carrying niggles a couple of days to relax and recover.
"We will get a good week's training under our belt ahead of what is a massive game against Motherwell.
"We have been playing well and not getting the wins so hopefully that comes on Saturday
"We want a good performs, but the result is huge for us. We weren't great last time against Motherwell (Well winning 5-1) but they are a good side in form and they really clicked that day at Firhill, James McFadden was back to his best.
"There is a bit of revenge on our minds because they did beat us handsomely."