PSG travel to Old Trafford to face Group H leaders United in Manchester on Wednesday, with all eyes on star Cavani.
Cavani joined United on a free transfer on deadline day in October, having left PSG as the Ligue 1 champions' all-time leading scorer with 200.
The 33-year-old Uruguay international won six league titles during his time at PSG, where he arrived from Serie A outfit Napoli in 2013.
Cavani has scored three Premier League goals for United after coming off the bench and netting twice against Southampton, including the 92nd-minute winner on Sunday, but his decisive display has been overshadowed by a social media post containing a Spanish phrase many would consider to be racially insensitive.
Tuchel was asked about the controversy which is being investigated by the Football Association (FA), and he hailed Cavani on the eve of the showdown.
"I'm surprised [by the social media comment], because I know Edi as a calm person, determined on the pitch to help the team, which is a rare characteristic for a striker of his quality," Tuchel told reporters.
"Always ready to do everything for the team. A big physical impact, scoring comes naturally to him and he's reliable 24/7 – you can call him in the night and he will be there – to score, to defend, to score, to defend.
"I'm happy for him personally that he could show this to all Manchester United fans in the last game against Southampton – he was absolutely decisive for them.
"He does not need to show us tomorrow, we already know, so he can do a little pause, it's not necessary to show us. If he will play, we will need to find answers for that."
Cavani became the first United player since manager Ole Gunnar Solskjaer in 1999 to have at least three goal involvements as a substitute in a single Premier League game as the Red Devils overcame Southampton 3-2.
United, who only require a point to reach the last 16, are three points clear of last season's Champions League runners-up PSG and RB Leipzig atop the table.
Since the start of the 2016-17 season, PSG have only failed to score in one of their 39 games in the Champions League, while they have netted an average of 2.4 goals per game in this run (95 in total) – the only side to keep a clean sheet against them in this run was Bayern Munich in last season's final.
PSG star Neymar has been directly involved in 60 goals in 63 appearances in the Champions League (36 goals and 24 assists) – since his debut in the competition in 2013, his 24 assists is the most of any player.