This is the quote:
“It’s just frustrating when you give up 46 points in the third quarter, and we’re just getting killed on the boards right now, and we don’t have any help-side where guys are contesting at the rim every single time,” said Irving, although the Celts lost the glass just 43-40 and had nine offensive rebounds to the Spurs’ eight. “That’s where we’re missing Big Rob [Williams] or a rim presence where guys are thinking twice about going in. I think about my drives, all in I shot a few floaters, but [Jakob] Poeltl’s there every single time. Not one time was he not there, and every time you think you have an open layup, they’re sliding over. And that’s just the defensive presence that you have to have having a five and a four like LaMarcus and Poeltl.”
So he says they miss Williams OR a rim presence i.e Baynes, basically they need another big in there is what he's saying, wether its Williams or Baynes. He probably doesn't mention Baynes because he's out for a while with a broken hand, but Williams could be back at any moment.
He clearly says that having Poeltl out there together with Aldridge is what makes it tough on opposing offenses, and the Celtics need someone like Williams or Baynes in there together with Horford or Theis, not instead of Horford or Theis.
This is a guy who was used to playing with Tristan Thompson for most of his career. It is reasonable that he wants another legitimate rebounder/rim protector in there with him.