The former Liverpool trainee, 25, has not made a first-team appearance for Hull this season but spent a month on loan at Carlisle.
Welsh joins loan striker Phil Jevons in bolstering Bury's promotion push.
"We've had to call in a favour to get John in but we've missed out on another player we've been speaking to this week," said Knill.
"We can easily say play-offs will be good enough but I don't think it will be now because we've got ourselves in a strong position so we're chasing automatic promotion.
"I don't know what points total we're going to need and we haven't set a target, we're just looking at this next game."
Defender Ryan Cresswell is available again after serving a one-match ban.
Bury league form: WDDDD
Bournemouth league form: DWWWL
Key opponent: Brett Pitman - Fiery forward will be a threat
Prediction: Bury 2 Bournemouth 1 - the Shakers have become draw specialists of late but this is a good chance to gain maximum points
Agree with the prediction? Have your say.
You can get 7/1 on Bury winning 2-1 with Betfred. For this and more odds, click here.
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