Bale is back at Spurs for a second spell and Mourinho says it cannot be assumed the loanee will make the same impact second time around.
He related the Welshman's situation to those of Lionel Messi and Cristiano Ronaldo, who have made subtle changes to their styles of play over the years to remain formidable forces.
Mourinho hopes Bale can add plenty to Tottenham too, but at the age of 31 it may not be as the explosive wide man of years gone by.
Asked after that game how his team for Brighton would look, Mourinho said: "Do you want me to start with the same team as tonight? You are an intelligent guy, you know my answer."
When addressing the matter of Bale, who had two shots – neither of which were on target – before being substituted after 58 minutes in Antwerp, Mourinho said: "I repeat, seven years is a long gap. Which player in the world is the same as he was seven years ago? Sometimes they are just different players.
"You look for example at Ronaldo and Messi and compare them to seven years ago. I think it happens with the majority of players. For sure, he is a different player."
Whether Bale faces Brighton remains to be seen. "He is working very hard. He is improving," Mourinho said. "He didn't miss one single minute of any training session. To play 90 minutes it's still hard, but he is improving. Hopefully he'll be back to a better level soon."
Tottenham have home advantage for the Brighton game and sit fifth in the Premier League heading into the weekend.
Mourinho is a tough taskmaster and will want an improvement on recent home form, Spurs having achieved all three of their Premier League wins away from the Tottenham Hotspur Stadium this season.
At home they have drawn twice and lost once, against Everton in their season opener. The last time they failed to win in four consecutive top-flight home games was in the 2013-14 season.
Their record against Brighton bodes well for that sequence to end, having won all three of their previous Premier League home clashes with the Seagulls - and there was good news from Mourinho as he forecast Eric Dier and Tanguy Ndombele should be fit for the game after physical ailments.
Having Ndombele back provides cheer for Mourinho, who said: "We missed him because he gives speed to the team, thinking quick, moving the ball fast. We miss him."