Andre Villas-Boas says Radamel Falcao had the same trouble as Wayne Rooney with penalties, and that he ultimately had to remove the Colombian from spot-kick duty at FC Porto.
But, in spite of Rooney's weakness from ten yards, Villas-Boas said the England international is one of the Manchester United players he fears most.
Should Alex Ferguson's side win a penalty in their match at Spurs side on Sunday, the forward will unlikely be the taker as a consequence of missing his 10th effort out of 28 in United's 1-0 win over West Ham on Wednesday.
Villas-Boas stated that Falcao, who is currently considered one of the most clinical strikers in the world, had similar issues in the past.
He said: "I don't know [why]. Falcao had the same problem. Maybe it's because they don't have the reference of a defender to beat, don't have anybody to anticipate. It's just them against the goalkeeper and the run to the ball, maybe it could be something like that."
When asked how he helped his forward, Villas-Boas merely said that he did not. He explained: "I just got another penalty taker. It was between him and Hulk at the time and he missed a couple - but he also scored a couple too, just like Rooney."
Even if Rooney will not be facing down Hugo Lloris from the spot, though, the forward remains one of the United players that Villas-Boas is most concerned with for Sunday's game. The Portuguese lamented the fact that the striker always seems to return from injury against him, as with the 3-2 win at Old Trafford in September.
He said: "We're always very unlucky as he seems to return just in time for our games. He played a second half [in September] and had a big impact.
"We recognise that we had a fantastic first half in that game but he came on in the second and made a difference and created lots of problems for us. He had this game against West Ham [on Wednesday] to come back to form and so we'll just have to wait and see if he is selected or not but I think he's a player who makes Man United tick.
"He always sacrifices himself for the team and he has team spirit, desire to win, and the focus. He has overcome some difficult moments in his career but he's a top-quality footballer."
Villas-Boas stated that the Rooney-Robin van Persie partnership is the toughest he has faced, concluding: "I think so. It is a very, very strong partnership at the moment. The individual talent is absolutely amazing."