McDermott admitted on Thursday that he was still on the lookout for new players despite adding three faces to his ranks this month.
Reading have monitored Aspas' progress since the Spaniard helped Celta achieve promotion last season with an impressive record of 25 goals in 37 matches.
The 25-year-old has made the step up to the Primera Division with ease this term, starring for Celta in what has otherwise been a tough return to the top flight for the Galician club.
With Chelsea and top Spanish sides reported to be interested in signing the player, McDermott appears to have been priced out of a move for the star, who is said to be valued at around £10million.
"I would be surprised if anyone could do that deal. It's an expensive deal and a difficult one for anyone to do," said McDermott.
"I've seen him play, our scouts have seen him play and he has had a couple of really good seasons.
"We know him - but it's a very difficult deal to do. Celta Vigo are a big club.
"The money that they are talking about on this player (Aspas) is a fortune.
"There would have to be some structuring to do on that - but we've never done a deal like that.
"The biggest deal that this club has done is £2.5million when we signed Emerse Fae and we sold him back for the same sort of money - the same with Greg Halford."
Despite admitting Aspas is highly unlikely to join, McDermott is still keen to add new faces to his squad to help him in his bid to keep Reading in the Premier League.
Stephen Kelly, Hope Akpan and Daniel Carrico have joined so far this month, but given that the Royals are still three points adrift of safety, their manager admits it is possible more new arrivals could be on their way.
"We have signed three, but we are still in the process of looking," said McDermott, whose team travel to Newcastle on Saturday on the back of a thrilling 3-2 win over West Brom last weekend.
"There are a lot of complications trying to sign a player as everyone knows, but if something comes our way, we have the finances and he wants to play here then we will do something. There is no doubt about that."