Beattie had scored twice on Saturday to cancel out Niko Kranjcar's opener for the visitors with Stoke seconds away from claiming three crucial points in their battle to beat the drop.
But Shawcross inadvertently diverted Hermann Hreidarsson's cross-shot beyond Thomas Sorensen in the closing moments as both teams were forced to settle for a share of the spoils.
Tony Pulis' men are just two points above the bottom three but January signing Beattie remains convinced they can beat the drop.
Home games"It's unfortunate for Ryan," Beattie told The Sentinel. "The shot was going about 20 yards wide and it's hit his shin and gone in.
"But you can't get too down about it. He's a very honest lad and will probably analyse that himself, but as I've already told him, I thought he was excellent again for us.
"We had three points cut to one, but at least we stopped Portsmouth from winning and getting further ahead of us.
"It's on the road where we have to start picking up points, but I haven't changed my mind about the chances of the club staying up.
"We have a lot of home games to come against teams around us where we can pick up a lot of points."
StickFormer Southampton striker Beattie came under fire from the visiting fans for much of the afternoon, but silenced their taunts with his first goal from the penalty spot before adding a close-range header.
He joked: "I think the Portsmouth fans love me and so I love them back.
"That goes way back to my days at Southampton and I thought they would have forgotten about that by now.
"They showed what they thought of me, so I thought I would show the same back. They are always going to voice their opinions about me. I have a good record against Pompey and some individuals take it a bit more seriously than others.
"If they want to give me stick, then that's up to them. They haven't grasped yet that it just makes me play better."