Coventry City have dropped to fourteenth in the League One table after matches were played on Tuesday night.
An 87th minute Porter goal was enough to give Sheffield United a 1-0 win at Colchester United and move them above the Sky Blues who are five points and a better goal difference clear of the relegation zone.
City could have dropped further had Oldham Athletic beaten MK Dons at Boundary Park but goals from George Baldock and Izale McLeod in the opening forty five minutes had the visitors ahead. Oldham, who had replaced former Sky Blues Danny Philliskirk and Jon Stead at the break, pulled a goal back through McDonald in the 57th minute but it was not enough.
Crawley could also have gone above City if they won at Swindon but they could only draw 1-1.
Preston North End and Rotherham United drew 3-3 and bottom side Stevenage beat Crewe 1-0.