Chelsea lost Antonio Rudiger (29) and Andreas Christensen (26) during the close season before signing Kalidou Koulibaly (31) from Napoli, but the Senegal international suffered from cramps and was withdrawn early in their 1-0 win over Everton last weekend.
"We could use one more central defender if you look at the numbers... we play with a back three," Tuchel told reporters, before addressing their need for a forward.
Chelsea's record signing Romelu Lukaku (29) re-joined Serie A side Inter Milan on a season-long loan, while the misfiring Timo Werner returned to Bundesliga outfit RB Leipzig. They brought in Raheem Sterling (27) from Manchester City.
"We could strengthen our squad in depth. It's one thing that you wish for and it's another that's possible. We're looking for top quality and top personalities," Tuchel said.
"We'll not panic and we will not try to sign players we're not 100% convinced. All signings so far are fantastic signings and will help us."
Chelsea, who have also recruited defender Marc Cucurella (24) from Brighton & Hove Albion, have been linked by the British media with moves for Leicester City defender Wesley Fofana (21) and Barcelona's Frenkie de Jong (25) and Pierre-Emerick Aubameyang (33).
Tuchel was reluctant to talk about players from other clubs but said he had fond memories working with Aubameyang at Borussia Dortmund.
"It was a pleasure," Tuchel said. "Some players stay your players because you were very close and Auba is one of these players," Tuchel added.
"Even when we played against him with Arsenal, straight away there was this close bond."
Chelsea take on London rivals Tottenham Hotspur on Sunday. The transfer window will close in England on September 1.