Sir Alex Ferguson's side are still chasing an unprecedented quintuple, having won the Club World Cup in Japan and the Carling Cup as well being seven-points clear at the top of the Premier League and still active in both the FA Cup and Champions League.
But ex-England defender Parker believes United may not have won so many games this season by the odd goal had they varied their set-piece deliveries, rather than relying on their Portuguese superstar to net from distance.
Ronaldo has netted several spectacular goals from dead-balls in the past, including a sensational set-piece against Portsmouth last season and the match-winner in the recent 2-1 success against Blackburn Rovers.
But he tried, and failed, to find the back of the net on several occasions at the San Siro against Inter Milan last Tuesday, with United's dominance over Jose Mourinho's Serie A leaders not resulting in a crucial away goal for the visitors.
And Parker believes that may not have been the case had they used their set-pieces more effectively.
"Alex Ferguson needs to have a word with Cristiano Ronaldo about his free-kicks," he told Eurosport.
"Against Inter, most of United's best chances came from set pieces around the box, which Jose Mourinho's side seemed happy to give away, knowing that Ronaldo would shoot every time, no matter how far out.
"He had that one right at the end that almost caught Julio Cesar out, but the rest of his set pieces made the goalkeeper look good.
"We've seen it before with Roberto Carlos, when he scored a couple of amazing ones that got shown around the world, but when you actually saw him taking them every week then he hardly ever scored. The same is true of Ronaldo.
"He got one against Blackburn, but how long before he buries the next one? Alex Ferguson needs to do something about that because it's really depriving his side of attacking options.
"In the first half at the San Siro, there was really only one side in it. Manchester United had all the possession, and were the only one's who looked like scoring.
"United's front two, Dimitar Berbatov and Ronaldo, couldn't take full advantage, and they will return to Manchester regretting that they did not kill the tie off or at least take the lead.
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