The Germany international has had an impressive first season in English football, scoring 14 times in all competitions.
"He had a good start [to the season], a very strong start," Wenger told arsenal.com. "After that, he suffered a little bit physically and mentally because in Germany they have a break in mid-season.
"We have seen with many German players that when they come in for the first season, mentally they suffer a little bit. Now he has recovered and is getting stronger again.
"On the flanks is the most demanding position. Physically, especially in England, it's very tough.
"He's a striker. Many times we were in a position where we were in a winning position in the final 20 minutes and then I take a bit more of a guarantee defensively and take him out of the team sometimes."