Izale McLeod had a penalty saved as Crawley suffered a third successive League One defeat as they went down 2-0 away to ten-man MK Dons.
The home side had former Crawley defender Kyle McFadzean sent off for bringing down McLeod when he was clear through on goal on 68 minutes.
However, McLeod’s spot-kick on his return to MK Dons was saved by David Martin.
The score was 1-0 at the time of the penalty, England under-18 international Dele Alli having opened the scoring on 15 minutes.
Benik Afobe added a second for the ten men in stoppage time.
Gregory made one change from the side that performed well when losing 3-1 at Norwich City in the Capital One Cup in midweek, Gavin Tomlin replacing Lewis Young.
The hosts made a strong start and threatened twice in the first six minutes. First, Tom Hitchcock played in Afobe but he could not get enough on his shot and Brian Jensen grabbed the ball at the second attempt.
Moments later Joe Walsh was dispossessed and Daniel Powell burst into the penalty area before shooting low against the far post.
It was a let-off for the Reds but the reprieve did not last long as MK Dons took the lead on 15 minutes, 18-year-old Alli shooting low into the far corner from 20 yards.
Crawley were immediately forced into a change, Bobson Bawling hobbling off to be replaced by Emmett O’Connor.
The hosts threatened a second on 20 minutes as Afobe’s shot on the turn got the slightest of deflections to arrow narrowly past a post.
After surviving the early onslaught, Crawley enjoyed a much better spell towards the end of the half, their best chance coming when Tomlin’s shot on the turn was deflected for a corner.
Five minutes into the second half O’Connor delivered a cross which Tomlin headed over.
Crawley had a golden chance to equalise on 68 minutes when McLeod burst through the middle and was brought down for a penalty by McFadzean who was shown a red card.
However, McLeod’s spot-kick down the middle was saved by David Martin and the hosts immediately countered, with Jensen saving well at the other end.
The action was now thick and fast, with Conor Henderson’s 20-yard strike being beaten away by Martin, then Afobe’s free-kick from the edge of the box for MK Dons struck a post.
Another Afobe free-kick moments later struck the Crawley wall amid strong appeals for handball which referee Mark Heywood waved away, then soon after Jensen saved well from a fierce Afobe strike.
With six minutes remaining O’Connor knocked Ryan Dickson’s cross back across goal but Tomlin’s shot was well saved by Martin, with George Baldock hacking the loose ball clear.
However, Afobe wrapped up victory for the hosts two minutes into stoppage time, shooting across Jensen into the far corner after making space for himself in the penalty area.
Crawley: Jensen; Oyebanjo, Leacock, Walsh, Dickson; Edwards (sub Harrold 78), Bawling (sub O’Connor 17), Smith, Henderson, Tomlin; McLeod. Subs not used: Banya, Bradley, Rose, Young, Ashdown.
MK Dons: Martin; Baldock, Kay, McFadzean, Lewington; Potter (sub Randall 54), Alli; Powell, Hitchcock (sub Carruthers 66), Bowditch (sub Spence 70); Afobe. Subs not used: Flanagan, Rasulo, Grigg, McLoughlin.