Manchester United's Van Nistelrooy, who missed last night's Premiership clash at Arsenal with an Achilles injury, was banned for three matches for a crude tackle on Arsenal's Ashley Cole in United's controversial 2-0 win in October.
"Like a lot of Dutch players, there is a darker side to him," said his Arsenal counterpart, who has himself been criticised for some poor challenges in the past. Bergkamp added that a nasty streak can often accompany a Dutchman's natural skill.
"I have had moments myself in my career. On one side he is a brilliant footballer but on the other a little bit dirty, like a lot of us. You find that many strikers have this mean side to them - a side which in some ways, they have to have. It helps them stand their ground but it can look clumsy."
Elsewhere, Middlesbrough manager Steve McClaren has admitted his disappointment at drawing a blank during the transfer window. Boro failed to persuade Celtic to part company with the central defender Bobo Balde, while Geremi turned down the chance to return to Teesside from Chelsea at the fourth time of asking. That has left McClaren still struggling with an injury list which is threatening his side's chances of building on their good first half of the season.
"We put a great deal of effort into bringing a couple of top-class players into the squad before the deadline, but unfortunately it wasn't to be," he said.
"We wanted to sign Bobo Balde, but Celtic weren't keen to let him go, while we agreed a fee with Chelsea for Geremi, but the player turned down our offer of a three-and-a-half-year deal. It's disappointing, of course, because we really wanted to bring in a couple of quality players to strengthen the team, especially with injuries to several key players."
With Gaizka Mendieta, Ugo Ehiogu, Chris Riggott, George Boateng and Mark Viduka having all been troubled by injuries, McClaren has found himself having to rely on his youngsters. "We have to concentrate on what we have got, ensuring we get our injured players fully fit as soon as possible," he said. "We are still in a great position in the league and have a very good squad that has got us this far. We must continue to strive to make the remainder of the season the success we believe it can be."