Lamptey's hamstring problem sustained in the 0-0 draw at Fulham has taken longer than expected to mend and he has missed the previous four Premier League matches against Sheffield United, West Ham, Arsenal and Wolves.
It was unlikely Lamptey would have been rushed back for Sunday's cup clash at Newport but the former Chelsea man now looks set to miss Albion's Wednesday night's Premier League trip to Manchester City as well.
Striker Danny Welbeck (knee), Adam Lallana (illness) and Steven Alzate (illness) are also expected to miss the trip to Rodney Parade. There was also bad news for Brighton striker Aaron Connolly who injured his hamstring after scoring in the first half of the 3-3 draw against Wolves.
Connolly will not feature in south Wales and looks set to miss Albion's next Premier League matches against Man City and Leeds
"Tariq Lamptey has had a little setback in his rehab," confirmed Albion head coach Graham Potter.
"Danny Welbeck, this game will probably come a bit too soon for him. Aaron Connolly, he will be out for a couple of weeks and Adam Lallana has been ill today, so I am not sure he will recover in time for the game. Steven Alzate is still recovering."
Brighton 17th in the Premier League and Potter admits he has some careful decisions to make for Sunday's FA Cup tie as he juggles the FA Cup with a relegation battle.
"We’re having those conversations," said Potter "We want to win the game against Newport, we want to go through, so it’s important we have that mentality and that approach.
"Then we have got a big week in terms of three matches in six days, so we have to be conscious of that.
"But we have to focus on Newport and take that game first, we will play a team that we think gives us the best chance to win."