The midfielder received three red and eight yellow cards last season for Italian club Udinese, who sold the 22-year old to Pompey for a reported £7.2million last month.
And Redknapp is worried his dismissal following a second bookable offence against champions Manchester United could signal a nervous time ahead.
And Redknapp admitted: "Muntari is stretching into challenges and has got to watch it. The first foul was an innocuous trip on Ronaldo but he should have been more careful with the second one.
"Referees in the Premier League will pick up on that and often they can't take account of over-enthusiasm.
"I think Sulley has a big future here but he has to be a bit more careful with his challenges."