Craig Beattie scored his fourth goal in the series and Scottish champion Celtic stunned Man United, 2-1, before 55,421 fans at Lincoln Financial Field in Philadelphia.
Spector was at fault on Celtic's goal, fouling Stilian Petrov for a penalty kick after having misplayed the ball.
Celtic, dominated by Chelsea and Liverpool in its first two ChampionsWorld games, was far crisper and better organized against Manchester United.
The match was the fourth in the seven-city, 11-game ChampionsWorld exhibition series.
Beattie, on the right flank several yards inside United territory, took a pass from Chris Sutton, eluded a sliding attempt by Roy Keane, then beat an onrushing Howard from the edge of the box for the winning goal in the 71st minute.
Beattie scored twice in Celtic's 4-2 loss Saturday to Chelsea in Seattle, and he netted a penalty kick in a 5-1 defeat Monday to Liverpool in Hartford, Conn.
Sutton also scored for Celtic, a ninth-minute penalty kick after Stilian Petrov was brought down in the box by Christopher Eagles.
Howard played the full 90 minutes. Spector, just 18, was pulled in the 85th minute for Luke Chadwick.
Alan Smith, acquired from Leeds United, scored his first goal in a Manchester United uniform, heading home Eagles' corner kick in the 35th minute.
Spector was sent into the Celtic box with the ball in the 80th minute, but two Celtic defender rushed in to cut off his angle to the goal.
Celtic plays its final game in the series Saturday in Toronto against AS Roma. Manchester United meets AC Milan on Saturday in East Rutherford, N.J.
July 28 at Philadelphia
Celtic (Scotland) 2 (Sutton pen. 9, Beattie 71) Manchester United (England) 1 (Smith 35)
July 29: Chelsea (England) vs. AS Roma (Italy) at Pittsburgh
July 30: FC Porto (Portugal) vs. Liverpool (England) at Toronto
July 31: Manchester United (England) vs. AC Milan (Italy) at East Rutherford, N.J.
July 31: Celtic (Scotland) vs. AS Roma (Italy) at Toronto
Aug. 1: FC Porto (Portugal) vs. Galatasaray (Turkey) at East Rutherford, N.J.
Aug. 2: Chelsea (England) vs. AC Milan (Italy) at Philadelphia
Aug. 3: Liverpool (England) vs. AS Roma (Italy) at East Rutherford, N.J.
July 24 at Seattle
Chelsea (England) 4 (Smertin 16, Gudjohnsen 49, Kezman 59, 87) Celtic (Scotland) 2 (Beattie 28, 68)
July 25 at Chicago
Manchester United (England) 0 Bayern Munich (Germany) 0
(Bayern Munich wins penalty-kick shootout, 4-2)
July 26 at Hartford, Conn.
Liverpool (England) 5 (Riise 5, Owen 17, Cisse 62, 76, Henchoz 68) Celtic (Scotland) 1 (Beattie pen. 72)
OTHER INTERNATIONAL CLUB MATCHES
July 29: Everton (England) vs. Pachuca (Mexico) at Houston
Aug. 1: Club America (Mexico) vs. Everton (England) at Houston
Aug. 7: Guadalajara (Mexico) vs. Santos (Mexico) at Dallas