The visitors gifted the Baggies two goals in their 3-1 loss at The Hawthorns, their ninth game without a win in the Premier League.
Chris Brunt, Gareth McAuley and James Morrison capitalised on poor defending to complete a comeback win for Albion.
Robert Snodgrass opened the scoring, but Hull fell apart after allowing Brunt a free header to equalise four minutes after the break.
The second-bottom Tigers are three points from the safety line and Phelan was left frustrated with their fragility.
“We have to be more courageous and braver and sometimes take a knock. You can’t just be a pretty boy, you have to get your body or something on it,” he said.
“When it is a one-on-one situation you have to carry out that duty. For 45 minutes that was probably as good as we have played.
“We got the goal and we were in control, but the game doesn’t last 45 minutes and we suffered again.
“You can’t win games by playing nice neat football, you have to do the dirty bits at the other end.
“There’s only so much we can prepare and the staff do it really well and the players are aware of that. After the game it’s annoying they didn’t carry those bits of detail out.”
Hull are without an away win in the league since August, with Snodgrass’s goal their first in the top flight outside the KCOM Stadium since October, and Phelan knows he needs January reinforcements.
“I’ll be having those conversations again, we have to look at it, we are now into the month so hopefully we’re not waiting until the 28th or 29th to do our business,” he added.
Albion are eighth following their second-best points haul, 29, after 20 Premier League games and they continue to mount an unlikely Europa League challenge.
This victory followed a 2-1 win at Southampton on Saturday and boss Tony Pulis rejected any talk of players being tired after two games in 48 hours. “If you tell people they’re tired, they’ll be tired – you don’t have to do too much to convince them,” he said.
West Brom: Foster, Dawson, McAuley, Evans (Nyom 30), Brunt, Fletcher, Yacob, Chadli (Robson-Kanu 62), Morrison, Phillips, Rondon (McClean 87). Unused substitutes: Olsson, Gardner, Myhill, Galloway.
Hull: Marshall ,Maguire, Dawson, Davies (Henriksen 84), Elmohamady, Livermore, Huddlestone, Clucas, Snodgrass, Mbokani (Mason 65), Diomande. Unused substitutes: Robertson, Meyler, Maloney, Jakupovic, Weir.
Referee: M Clattenburg (Tyne & Wear).