Investors are not fully prepared for a September rate hike, and they're too complacent about the chances for another one in December, according to money manager Jeroen Blokland.

"The U.S. economy has continued to grow showing decent numbers," said Blokland, senior portfolio manager at Robeco, on Tuesday's "Futures Now. "I think the Fed just has to get off the 0.5 percent at some point and this is probably a good time to do so."

Blokland, who has just under $300 billion in assets under management, believes the Fed has already missed a couple of vital opportunities for a rate hike.

"The Fed should raise rates sooner than later," he contended. "Jobs have grown at an above-average pace since December. Wages, income, spending are all growing at a decent pace. Consumer prices have risen as well."

If Friday's August employment report is strong, it'll give the central bank more ammunition to boost rates a quarter point to 0.75 percent.

"We have to take into account that these interest rates are still extremely low and that the Fed is looking for a gradual path to normalization," he said.

The Fed has not raised rates since last December's meeting. Since then, the is up 6 percent — while gold and oil have soared 24 percent and 28 percent, respectively.

On Friday, Fed Chair Janet Yellen said the case for an interest rate rise "has been strengthened" by recent data, though she did not lay out a timetable for future hikes.

And as for stocks?

"The markets look a bit frothy. So, I think this could be an important hurdle for stocks to rise further from here," Blokland said.