Gray equalised twice in the 2-2 draw against his former side Watford at the Home Park stadium, salvaging a much needed point as Argyle attempt to distance themselves from the Championship relegation zone.
In the 22nd minute, standing on the sidelines, Rooney may have had a flashback to April 2013.
Lewis Gibson, a few yards past the halfway line, chipped a long ball for Gray to hit as it came down over his shoulder past a helpless keeper.
It was reminiscent of the match against Aston Villa at Old Trafford, when Rooney launched a precise ball from just behind the halfway line to Robin van Persie, who struck it perfectly on the volley, sending it soaring past Brad Guzan.
And Gray's second goal, deep into second-half stoppage-time, was just as good.
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The 33-year-old collected the ball on the edge of the box in the dying seconds and curled a stunner into the top corner, sparking wild scenes amongst the home faithful.
Rooney said: "I was confident we could get a late goal, because we've done it a few times now and there is that belief, especially when the fans get behind you.
"They've got the best player in the league but I thought after the first 10 minutes and going a goal down we did well to get back in it.
"We spread our centre-backs out a bit more in the second half and played better.
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"We created a few more chances and the lads who came on made a difference."
Gray joined the Hornets in October on a deal that runs until January and Rooney is confident the club will talk to the striker about a longer stay.
The former England captain added: "It was a fantastic goal from Andre. It's really difficult to score like that, a ball over the shoulder, I know how difficult that is to take, and it's a world-class finish.
"Andre's second goal was a fantastic finish.
"We have got a lot of games coming up between now and January and you can see the quality he has got.
"I am sure before January there will be conversations about him staying."
Meanwhile, at Man United, new manager Ruben Amorim will hope to resurrect the success of Rooney and Van Persie's era following a turbulent decade.
Amorim has a huge task on his hands to unite the club - a job made harder by injuries, meaning he will likely field a makeshift backline at Portman road.
He is the second Portuguese to take charge at Man United following the legendary Jose Mourinho.
At Friday's press conference, Amorim revealed that Mourinho sent him a message.
"He sent me a message, it's a big club, lovely club with lovely people, and he's correct, it still is," Amorim said. "We are building a new club.
"I am a different guy. I hope to teach something different here to my players. It's the best club in England. We want to win, that's all."
His philosophy and style of play may take a while for the players to learn, especially as it is heavily rumoured the formation will shift to a 3-4-3.
Speaking on his tactics, he explained: "As a coach, you have to choose one way or another. I choose always 100 per cent our way. I prefer to risk a little bit but to push in the first moment.
"If they feel that since the first day that I believe so much in our way of playing, they will believe too. So, there is no second doubts, no second way. It's one way and we are going to do it.
"We will adapt some players because we don't have the right profile. This team was built for a different system.
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"But it's the same thing playing with five or four. The principles are the same. The positioning is a little bit different.
"Maybe on Sunday when you will see the list of players, the starting 11, you don't feel a lot of change. But you will feel it in the game in the positioning, in the way they receive the ball or to cover the ball. You see some changes."