Chelsea appeared to be on course for a second consecutive Premier League victory after Abraham scored twice for the second week running to put his side 2-0 up at half-time.
Sheffield United rallied in the second half, though, with Callum Robinson scoring his first goal for the club shortly after the restart, capitalising on poor defending from the hosts.
In a bid to defend Chelsea’s 2-1 lead, Lampard opted to withdraw Abraham and Kovacic after 84 minutes with Michy Batshuayi and debutant Billy Gilmour introduced in their places.
Lampard’s decision backfired, though, as Sheffield United scored a deserved equaliser in the final minute of normal time when Robinson’s cross was turned in by Kurt Zouma.
When asked whether his decision to introduce Gilmour was bold, Lampard replied: ‘It wasn’t bold because it was one for one in both position.
‘Kovacic was on his haunches, tired, Billy Gilmour is a midfield player I have on the bench and Batshuayi on to bring some energy for Tammy, who had been our best player, clearly, in the game.
‘Other people can make what they want for that, but we were trying to see the game out.’
Chelsea have looked shaky at the back in the opening weeks of the season and Lampard sought to remedy that by starting Fikayo Tomori in place of Andreas Christensen in central defence.
The switch didn’t have the desired affect, however, and the result means that Chelsea have conceded nine goals in their first four league games – only Norwich City have let in more (10).
Lampard defender his back four, saying: ‘It’s not defence, it is conceding as a team. The lack of concentration and mistakes for the goals are moments, you defend as a team as much as you attack as a team.
‘We conceded because you switch off in a game that should be comfortably seen off at 2-0. That is no disrespect to Sheffield United, 2-0 is not the end of the story.
‘I was clear with the players on that at half-time, this can either be 3-0 and nice or a potential 2-2.
‘The disappointing factor is the first goal more than anything because it allowed it but we only have ourselves to look at as a group and not individually.’