A centre-half is top of manager Paul Jewell's wish list, while Whelan accepts other areas of the squad need attention too.
He told the Evening Post: "We still have another signing or two to make, with a centre-back particularly important.
"There could be a chance of getting someone in before we play Nottingham Forest and that is our aim.
"But transfers are not easy to push through and sometimes you have to be patient before they come off.
"Paul is fortunately a manager who is prepared to wait and only brings in who he wants – he doesn't settle for second best.
"As well as a centre-back, we do require a bit more cover in the squad.
"Last season we were three, four or five players short, and that did cause us problems.
"Over the summer we have brought in three signings so far, so there is still some building to be done.
"Gradually, Paul is getting to the point where he has the team and the squad he wants."
Latics' main target to fill the defensive vacancy is Blackburn's Andy Todd, a player they unsuccessfully bid £250,000 for in May.
There have been signs Rovers are ready to drop their asking price below the £500,000 they originally wanted.
They made a move for Rangers' Craig Moore over the weekend, which would leave Todd surplus to requirements at Ewood Park.
Even with a strengthened squad, Whelan refuses to make any rash predictions about Latics' chances this season.
Promotion to the Premiership remains his long-term aim, but for now he is content with a top-half finish.
A cautious Whelan said: "The season coming up will be an exciting one and the division is a very good league. There are some big clubs in there – Leeds were in Europe not so long ago – and I think our aim to start with has to be to finish in the top half of the table.
"I'm not going to get carried away by saying we want the title or a play-off place.
"The aim is still to get into the Premiership one day but we have to be realistic and take a step at a time.
"During a season a lot depends on luck and how the referees look at you, things like that.
"I think we have a very good side and the performance at Blackpool on Sunday, albeit in a friendly, was superb.
"Blackpool might have been from the league below, but it's still good to win these games.
"The pace up front was electrifying, Jimmy Bullard controlled the midfield and Per Frandsen was excellent.
"Frandsen has settled in straight away here, and the other two signings – David Graham and David Wright – are looking good too.
"With the friendlies out of the way, the acid test is now Saturday's opening game against Forest.
"That's a great game to start with and lets hope we can get 14,000 or 15,000 in the ground to see it."
* Season tickets will not be on sale on Saturday at the JJB, allowing the ticket office staff to concentrate on matchday sales for the Forest game.
They will be back on sale next Monday.