The 19-year-old had played only three senior games prior to his arrival at the Keepmoat from Bolton Wanderers last month.
But he will have an equal opportunity to that of Evina to stake a claim for a starting shirt during the coming season.
“There’s some development in the boy but he’ll be challenging with Cedric,” Ferguson told The Star.
“He ticks a lot of boxes. He’s got good pace, a good left foot, he’s a good age and a good size.
“Quite a lot of people at Bolton were surprised they’d let him come to us but we did our business early with him in terms of making our interest known.”
The Bolton News reported Rovers had paid £25,000 for Garrett, much to the annoyance of some Trotters supporters who had been impressed with his appearances which came following the confirmation of the club’s relegation to League One.
Garratt has been on Rovers’ radar for almost a year but interest in signing him increased following the knee ligament injury to Aaron Taylor-Sinclair which will keep him out for the rest of 2016.
Ferguson said: “We were looking around because we knew before the last game of the season that Aaron had the injury.
“We wanted to concentrate on getting a left back in and Tyler was one that was on that list.
“We thought it might be hard because he started the last few games for Bolton in the Championship.
“It went on a bit longer that we wanted because Phil Parkinson had only just gone in there at Bolton.
“But we managed to get it done and we’re delighted.
“He’s settled really well and I think he will be a good asset for the club.”
Both Garratt and Evina are likely to receive a full 90 minutes each in Rovers’ final two friendlies against York City and Coventry City next week after playing a half each in the previous three pre-season matches.