Former QPR and Preston frontman Agyemang, currently on a month's loan at Fratton Park, helped Pompey make it two games unbeaten with a fine display in the 1-1 draw with Carlisle on Saturday.
"He's a decent footballer, he's been around, has a lot of experience and we are glad to have him," Whittingham told The News.
"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.
"You need that in this league or League Two - you need that type of player, so it was vital we got someone in.
"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."