He played in both recent training matches against Germany's Under-20 team at their training base in Italy and is set to play in their friendly international against Austria on Saturday.
"Manuel will go to the World Cup as our number one," Bierhoff told reporters at their training camp in Eppan, northern Italy. "If he makes the squad then Marc-Andre (ter Stegen) will be number two."
Coach Joachim Loew has picked four keepers, including Barcelona's ter Stegen, Paris St Germain's Kevin Trapp and Bayer Leverkusen's Bernd Leno, for his 27-man preliminary squad, preparing in the Italian Alps.
He will cut four players, including one goalkeeper, on Monday for his final 23.
"Manuel is already full on track. He does not need to get back on it. All the players have left a big impression here and it is as very difficult decision," Bierhoff said.
"On the evening before the team announcement there will be the final thoughts among the coaching staff. By noon the next day the players will be informed."
The Germans also expect Chancellor Angela Merkel to drop in on their camp this week.
"My information is that on Sunday she will drop by. Before our training camp we visited her and she had hinted that she wanted to say hello without too much fanfare," Bierhoff said.
"It's kind of a tradition. It is nice to hear from her and how she sees the team's situation going into the tournament."
The Germans, who also face Saudi Arabia in Leverkusen on June 8 in their final warm-up match, are in World Cup Group F with Sweden, South Korea and Mexico. The tournament starts on June 14.
(Reporting by Karolos Grohmann; Editing by David Holmes)
