A scrappy Maxime Biamou goal 16 minutes into the second half ensured Dan Micciche's reign as MK Dons manager began with defeat, going down 1-0 to Coventry City.
Outnumbered in their own stadium by travelling Coventry fans, Dons' defensive frailty was on show as Biamou's effort dribbled over the line to send the Sky Blues into the fifth round.
There were positives to take for the home side, with a strong opening period, but with their best chance coming in stoppage time, Micciche will know he has a lot on his plate.
Dan Micciche named Dean Lewington in his first starting line-up, bringing the skipper back after nearly three months out, exiled under former boss Robbie Neilson. It was a much changed team to the side which lost to Northampton a week earlier, with Ouss Cisse in at centre half, Chuks Aneke suspended and Ryan Seager's loan deal terminated. It was a return to the 4-2-3-1 formation made famous under Karl Robinson, with Kieran Agard leading the line.
And the Micciche era began very much on the front foot as they dominated the opening 20 minutes. The attacking trio of Sow, Tavernier and Ugbo were pulling all the strings for the home side, but any questions about Lewington's inclusion were answered early on as he looked fit, sharp and back to his best against the League 2 side.
But despite the one-sided nature of the game, visiting keeper Lee Burge would barely be tested, and City would have the clearest chance on 25 minutes as Dons, who still looked shaky in the heart of the defence, allowed Maxime Biamou a sight of goal and he drew a fine save from Lee Nicholls.
Burge would have to be alert on the stroke of half time though as Tavernier took on three Coventry defenders to bear down on goal, only to have the keeper swoop in at his feet to deny him and send the sides in goal-less at the break.
The second half picked up exactly where it left off, with Dons seeing more of the ball but failing to really test Burge. Kieran Agard, having played out a lonely opening 45 minutes, should have put the home side ahead when he was picked out by George Williams' ball from the right flank but he somehow swiped over the top of the ball.
Lewington's afternoon came to an end early on in the half when he was removed for Ethan Ebanks-Landell, and after that defensive shake-up, Coventry capitalised and took the lead on 61 minutes. Dons were at sixes and sevens at the back as a harmless looking ball rolled across the box, and Maxime Biamou was adjudged to have got the final touch to take it over the line.
After that, Coventry had the upper hand as the home side had the stuffing knocked out of them. All of the slick, quick and incisive football was gone, hindered by Coventry's solid two banks of four as they sat back and smothered the home side.
Even with six minutes of stoppage time, Dons struggled to get anything clear cut in front of goal. Burge made his first real save of the game in stoppage time to deny substitute Aidan Nesbitt, though the visitors could have made sure but for Nicholls keeping out Ben Stevenson late on.
Referee: Peter Bankes
Attendance: 14,925 (7,833)
MK Dons: Nicholls, Williams, Lewington (Ebank-Landell 56), Wootton, Cisse, Upson (McGrandles 76), Gilbey, Sow, Tavernier, Ugbo, Agard (Nesbitt 65)
Subs not used: Siestma, Muirhead, Rasulo, Brittain
Coventry: Burge, Griummer, Stokes, Willis, McDonald, Kelly (Stevenson 76), Doyle, Biamou, McNulty, Barrett (Shipley 59), Bayliss (Kelly-Evans 82)
Subs not used: O'Brien, Vincenti, Hyam, Ponticelli
Booked: Kelly, McDonald, Ebanks-Landell, Tavernier