"I do not know who is fit and who can play," Schaaf told a news conference on Tuesday an hour before his side were due to train at the Sukru Saracoglu stadium.
"I am not even thinking about a (team) formation yet."
Schaaf said Diego, Almeida and Mertesacker "cannot be replaced on a one-to-one basis" but he would still be able to field a strong team.
Brazilian Diego, who has scored six UEFA Cup goals in Werder's run to the final, has been heavily linked with a move to Juventus with German media reporting on Wednesday the deal was worth about 29 million euro.
Werder general manager Klaus Allofs said a transfer was "still up in the air despite weeks of talks" while Schaaf told reporters Diego had not yet played his last game for the club.
"We still have an important league match against (Bundesliga leaders) Wolfsburg and then the German Cup final (against Bayer Leverkusen)," he said. "Diego will play a part in these games".
"If we win a trophy then the season will have been a success," he said. "We are not happy with our Bundesliga form but we have the chance to make up for that."
