The experienced forward, who is currently on a one-month loan deal, impressed for Pompey in their 1-1 draw with Carlisle United at the weekend.
"He's a decent footballer, he's been around, has a lot of experience and we are glad to have him. We knew it was going to be a battle against Carlisle - they have two big centre-halves who like heading the ball, have good strength and play well. But we are pleased how Pat played against them," Whittingham told The News.
"It's surprising when you find out where some players go when they are in the Premier League one minute then here they are in League One. He's a great guy. He's quieter probably than most people think because he puts himself about on the football pitch, but he's a really nice guy.
"Pat's here for an initial month but if he does well - and Stevenage are happy - hopefully we will keep him until the end of the season."
Agyemang has also played for Queens Park Rangers.