For someone who trained as an amateur boxer, the slap was one of the very softest Wayne Rooney has ever delivered, but the fact that it connected with the face of Tal Ben Haim meant it obscured everything else about this Lancashire derby, even another sublime display from Ryan Giggs.
Although it was missed by the referee, Dermot Gallagher, it is unlikely to be ignored by the FA when it convenes later in the week and with Ruud van Nistelrooy and Louis Saha injured, Manchester United can ill afford a suspension. With United now in fierce pursuit of Chelsea and Arsenal, Alan Smith is likely to begin the year as he started the season, as the only fit and available front-line striker.
Sir Alex Ferguson, questioned by the club's official television station, dismissed the incident out of hand, all but accusing the Israeli of throwing his face at Rooney's open palm. "It's embarrassing, the Bolton player should be ashamed," said the Manchester United manager of Ben Haim's theatrical writhing. "We had a referee who let things go and his performance was one of the best I have seen."
Although conceding that Ben Haim behaved like a very light featherweight confronted by Lennox Lewis, Sam Allardyce was less inclined towards generosity: "Yes, he went down rather too easily, but there is no doubt Wayne Rooney pushed him in the face and under today's rules he should have been sent off."
The Bolton manager did not share Ferguson's warmth towards Gallagher, especially his disregard of a venomous tackle from behind delivered by Roy Keane on El Hadji Diouf which merited not even a quiet word and ensured Diouf kept his distance from the United captain thereafter.
Keane is a player who can smell vulnerability and a Bolton side careering towards their fifth successive defeat stank of it. After picking up two points from a possible 21, Allardyce had embraced radical surgery, dropping Jay-Jay Okocha and Bruno N'Gotty. Instead, he invested his faith in Ricardo Vaz Te, an 18-year-old signing from the Portuguese side Farenese making his first Premiership start. There were a couple of sweet touches but his main contribution was to be struck in the face by an Ivan Campo drive that represented Bolton's best chance of the match.
This has been a month when the old heart of Manchester United - Giggs, Keane and Paul Scholes - has been beating strongly. Yesterday Giggs woke up to headlines suggesting that Manchester United might resolve the impasse on an extension to his contract that has 18 months to run by offering him a performance-related deal. On the Welshman's recent form, this could make Giggs very wealthy indeed. The passing, the running, the sheer lovely confidence of his game were absolutely undimmed.
His 125th goal in a United shirt was spectacularly consummated; falling backwards to connect with a cross from Gabriel Heinze that was meant for either Rooney or Cristiano Ronaldo but which missed them both. He ought to have had another when put clean through by Liam Miller but Jussi Jaaskelainen held his nerve - which is rather more than his defenders managed.
Allardyce thought the way they dealt with Heinze's cross for United's opening goal was unforgivably poor. However, there was little they could have done to prevent Scholes's seventh goal in as many Premiership matches, which was absolutely typical of a man whom Thierry Henry rates as the best English footballer in the world. It was brutally struck from the edge of the area, from a finely timed Giggs pass, and struck the back of Jaaskelainen's net with a minute remaining.
For Allardyce what had threatened to be a tilt at European football is now turning into what his hitherto hugely successful stewardship of Bolton was always about, keeping Premiership football at the Reebok Stadium. For a former centre-half it must hurt deeply that the deficiencies are where he used to play. "We need a minimum of 11 to 12 clean sheets in a season and so far we have had two in 19 matches," he reflected. "When we were winning games 2-1 it did not seem to matter but I always told the players that a reckoning would come."
Goals: Giggs 10 (1-0), Scholes 89 (2-0).
Manchester United (4-4-2): Carroll; O'Shea, R Ferdinand, Silvestre, Heinze; Ronaldo (Scholes, 68), Fletcher, Keane, Giggs; Smith, Rooney (Miller, 85). Substitutes not used: Howard (gk), P Neville, Bellion.
Bolton Wanderers (4-3-2-1): Jaaskelainen; Hunt, Haim, Jaidi, Cesar; Nolan, Campo, Speed; Vaz Te (Giannakopoulos, 68), Diouf (Okocha, 51); Davies (L Ferdinand, 62). Substitutes not used: Oakes (gk), N'Gotty.
Referee: D Gallagher (Oxfordshire).
Man of the match: Giggs.