Sule lauded his team-mate – who netted 29 Bundesliga goals in 2017-18 – for staying focused amid the speculation during the season.
"In the time he played for Bayern Munich, Lewy always scored his goals. He is a very important player for us," he told a news conference on Wednesday.
"Throughout the season there were many rumours about him, but we all stayed calm, Lewy as well. From the outside there is always more talk than it actually is inside the team.
"He was always motivated and scored again 30 goals this season in the Bundesliga. I don't think anyone should say his head was somewhere else than with the team.
"What his agent said or what the situation is with Lewy – only he knows best. But the club will handle this situation.
"All I can say is that Robert Lewandowski is highly respected within the team, he is a great guy and he always scores goals for us."
Lewandowski, who is contracted until 2021, has scored at least 40 goals in the past three seasons, having joined Bayern from Borussia Dortmund in 2014.