Costa played his part in Leeds crushing Southampton 3-0 at Elland Road on Tuesday night as the Whites broke into the top half of the Premier League table.
The winger was brought on off the bench at half time in place of Jack Harrison and Leeds scored three second half goals without reply.
Whelan was impressed with what he saw of Costa, but feels the winger is infuriating because while it is clear what he is capable of, he does not produce it on a regular basis.
“He is probably one of these infuriating players because you know how much quality he’s got, you know what he can do, but it’s so hit and miss with him”, Whelan said on BBC Radio Leeds following the game.
“If you can just get that consistency with him then £17m would look like a snip.
“But unfortunately there’s just too many inconsistent performances from him.
“We know he can rip people to shreds on the right hand side he’s got that much quality, but it’s just finding that consistency.
“That’s a difficult thing as a football player when people have that much talent because when you have a good game, people expect it every week.”
Costa has clocked 16 appearances in the Premier League for Leeds so far this season, chipping in with two goals and three assists.
The winger will hope to have done enough in his 45 minutes against Southampton to get the nod to feature against Aston Villa at the weekend.
