Valdes's injury comes at a pivotal time in the season for Boro, who sit six points from safety with six games remaining, but Agnew has no qualms with Guzan replacing the Spaniard.
"Brad Guzan has been great this season. He was in earlier on in the season and performed very, very well and again I thought he was very, very competent the other night," he told reporters.
"We have to remember he's played seven, eight years in the Premier League with Aston Villa, so he's an experienced goalkeeper.
"They sit and watch videos of the opposition and they know exactly the strengths and weaknesses of Bournemouth in an attacking sense. It's the experience that Brad brings - and Dimi, obviously - that gives you confidence."
Guzan has kept two clean sheets in his four league appearances this season.