The 27-year-old Argentine has done little to justify the British record transfer fee of 59.
7 million pounds United spent to recruit the flying winger from Real Madrid.
Di Maria, who has scored three goals in 19 Premier League games this season, was taken off at halftime during United’s 2-0 victory over Sunderland on Saturday but Rooney believes the Argentine will come good at Old Trafford.
“It (Saturday) was just one of those days that every footballer has,” Rooney told the club’s website.
“He is a great player, he has shown that and you don’t lose your talent overnight. Hopefully, he will get back to his best and start giving some great performances for us.
“He has got a lot of experience and I think he will figure it out, we all have to do that sometimes. There is nothing worse than when everyone keeps going on at you.
“You have to come to it in your own way and I am sure he will be a big player for us between now and the end of the season.”
Rooney has been deployed as a midfielder by manager Louis van Gaal for much of the season but he struck twice against Sunderland to end a run of eight games without a goal.
“Everyone knows I’ve played midfield for the majority of the season but my record shows that I score goals so I’m not worried about that at all,” he said.
“I was obviously pleased that the manager played me up front and I was delighted to get the two goals but the victory was the main thing today because after last week (when United lost to Swansea) it was important that we bounced back.”
United, who are fourth in the Premier League table with 50 points from 27 matches, travel to Newcastle United on Wednesday.
(Reporting by Michael Hann, editing by Ed Osmond)