The Pole struck just after the hour on his first appearance for the Bundesliga champions against MSV Duisburg on Monday following his Bosman move from Borussia Dortmund.
Lewandowski was one of a small handful of first-team regulars involved in the friendly that ended 1-1, and Bayern's sporting director Sammer was happy with a display, which saw Lewandowski test both goalkeepers used by the third-tier side.
"He's an outstanding player with outstanding skills," said Sammer. "We'll build him up slowly.
"[Monday] he played almost a full match.
"We've had to manage the training carefully this week because at the weekend we'll play a whole tournament.
"A lot of players are still in holiday. The players that are here cannot play every single match up to full time.
"We have to be careful but we also have to be at 100 per cent when the Bundesliga starts. That's quite difficult."
The player himself is looking forward to linking up with Bayern's World Cup winners having left the match with mixed emotions.
"On the one hand I'm happy to have scored my first goal but on the other hand we only managed to draw 1-1," he said. "Matches like this are more difficult than matches with the whole team together.
"Of course it would be better having all national team players here but we'll have to wait for them, that's a normal situation for a team with so many World Cup winners.
"I'm focusing on training and when the players come back we'll start the season. When they're back it would be another type of preparation I think."